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Where to start
Introduce cats slowly, in stages. Give the new cat a base room of their own, swap the cats' scents, then let them see each other through a gate or a door held slightly open, and only then try short, supervised meetings. Move on only when both cats are relaxed at the current step. It can take anywhere from a few days to a few months.
Before the new cat arrives: a vet check and a base room
Start with the vet. The cat vets at the AAFP say every cat in the house should have had a recent check-up, and the new cat should be tested for common contagious diseases before the cats meet.
Then get a base room ready: a quiet room your resident cat doesn't use. It needs food and water, a litter tray placed away from them, a bed, places to hide, somewhere high to sit, a scratching post and a few toys. International Cat Care says these things should be brand new or already the new cat's own, because using your resident cat's belongings can cause stress.
If you'd like to try a plug-in pheromone diffuser, the AAFP suggests one in the main living area and one in the base room, 24–48 hours before the new cat arrives.
Let the new cat settle in first
Carry the new cat straight to the base room, open the carrier and let them come out when they choose. For now, the cats live apart. International Cat Care says settling in can take from several days to a week or two.
You can tell a cat has settled when they greet you as you come in, rub their cheeks on the furniture, play, roll onto their side, and eat, drink and use the tray as normal. A cat who still hides all day isn't ready for the next step. Meanwhile your resident cat keeps their usual routine and their share of fuss.
Swap scents before they see each other
Scent comes before sight. Battersea suggests starting after two or three days: put a blanket from your resident cat in the new cat's room, then swap one the other way. A few treats next to it help.
You can also stroke one cat's cheeks with a soft cloth and leave it in the middle of the other cat's floor, so they can sniff it or ignore it. The RSPCA adds one rule: collect scent only when the cat is relaxed, or their stress ends up in the scent.
Once the scent no longer gets a reaction, let the new cat explore the rest of the house while your resident cat waits in a favourite room with treats. If either cat hisses at the blanket, growls, avoids it or pees outside the tray, stay at this step a few more days.
Food and play on either side of the door
Next, give the closed door a good meaning. The RSPCA suggests hanging a toy or offering tasty food on both sides of the door, so every cat links that room with something nice. The AAFP's version is a double toy: two toys tied together with strong string, one on each side. Play for 5–20 minutes and stop before the cats lose interest.
If one cat hisses or growls at the door, don't tell them off. The AAFP says to stop, put more distance between the cats and try again later. If your resident cat camps by the door or keeps pawing at it, put a second barrier in front of it, such as boxes or chairs.
The first look: through a gate or a door held ajar
When both cats are relaxed about the scent and the door, let them see each other but not touch. International Cat Care lists a glass door, a door held ajar so the cats can't pass, a stair gate or a mesh barrier. Battersea opens the door slightly and secures it. If you use a gate, the AAFP suggests an extra tall, door-sized one.
Keep these sessions short and calm, with treats or a game for each cat, and let each cat choose whether to come closer. Going well looks like the cats glancing at each other now and then without staring each other down. Hissing, growling, ears turning and a tense body mean the step was too big. Go back a stage for a few days.
Short, supervised meetings in one room
After plenty of calm sessions at the barrier, open it while the cats are busy with something else. Start with a few minutes, stay in the room, and make it pleasant with treats and toys. Then add time slowly. Cats Protection says both cats need to know where they can leave or get up high, so keep the base-room door open as a way out.
Never carry one cat to the other or hold either in place. Each cat should be free to come closer or walk away. They don't need to touch noses or play together. Two cats sitting in the same room and getting on with their day is a good meeting.
How long does it take to introduce cats?
It depends on the cats. The AAFP says it can take anywhere from a few days to a few months, and sometimes longer. The RSPCA gives a week to several months, and Cats Protection says the whole process could take weeks.
Let the cats set the pace. Move on only when both are comfortable at the current step, and go back one when they aren't. The AAFP warns that pushing ahead before the cats are ready can leave long-term tension between them.
Signs it is going well, and signs it is not
Going well can look boring. Battersea calls two cats ignoring each other a very good sign, and says learning to tolerate each other is a healthy outcome too. Many cats never become close friends. The friendly signs Cats Protection lists are grooming each other, rubbing past each other and sleeping touching.
Tension is often quieter than a fight. Watch for one cat blocking the way to the litter tray, the food or the cat flap, long stares, low growls, a cat leaving the room when the other walks in, or one cat pushed off the best resting spots. Play is usually quiet, and the cats take turns chasing. A fight has flailing front paws, flat ears, growling or hissing.
Hissing, growling or a fight: what to do
Mild hissing or swatting isn't unusual, Battersea says. If the growling keeps on or one cat turns aggressive, pause and go back a step. Don't punish either cat. International Cat Care says it only frightens them more.
If a fight breaks out, keep your hands out of it. Slide a large piece of cardboard or a cushion between the cats to block their view, or distract them with a toy or a loud noise. Give them time to cool off before you stroke them or pick them up. After a fight, Cats Protection's advice is to separate them again and repeat the steps more slowly.
If the fights keep happening, Cats Protection says to contact a qualified cat behaviourist as soon as possible. If a cat who used to be relaxed turns aggressive, see your vet first.
One of everything per cat, plus one
The rule Cats Protection gives for two cats or more is one set of the basics per cat, plus one extra, in different places. That covers litter trays, water bowls, beds, scratching posts, high perches and places to hide. For two cats, that means three litter trays.
Spread them around the house so each cat can reach their own without competing. Battersea suggests one food station per cat, placed apart, since cats don't naturally choose to eat close together, and water in several spots away from the food. The AAFP says the most common cause of tension between cats is not having enough resources, set apart and out of sight of each other. In a narrow hallway, shelves or a cat tree give one cat a way around the other.
How to introduce a kitten to an adult cat
A kitten can be easier. Cats Protection says your cat may not see them as much of a threat, and the time between steps may be much shorter. The steps are the same, with a few changes.
Check that the gaps in any barrier are too small for a kitten to squeeze through. Play with the kitten until they're tired, so their energy doesn't land on your older cat; the AAFP suggests two or more play sessions a day, of at least 5 minutes each, for a lively cat. Give the adult high places the kitten can't reach yet, keep their routine, and feed the two apart. They'll likely eat different food, and kittens steal.
Battersea's warning is about age and energy: a lively kitten may overwhelm an elderly cat.
Notes from the first week at the gate
Day 8. Tall gate in the spare-room doorway, Mia on one side, Leo on the other, three metres apart. Leo looked, blinked, ate his treats and washed. Mia watched from the windowsill, then came down to the gate and sniffed. Four minutes. Closed the door while both were calm.
Day 9. Same again. Leo stared for a long time, tail flicking. Ended it after a minute. Back to blanket swaps for two days.
A note after each session shows when a step was too big, so the next one can be smaller.
When it does not go to plan
What is the best way to introduce cats to each other?
Slowly, in stages, at your cats' pace: a base room, swapping scents, food and play by the closed door, a first look through a barrier, then short meetings. Each step lasts until both cats are relaxed. Cats Protection, Battersea, International Cat Care and the RSPCA all describe much the same order.
Can you introduce cats quickly?
Not really. The AAFP's range runs from a few days to a few months, and the RSPCA's from a week to several months. A kitten and an adult can move faster, Cats Protection says, but they still go through every step.
How do you introduce cats when one is aggressive?
Go back to the last step where both cats were calm and stay there longer, with more distance and shorter sessions. Check there are enough litter trays, bowls and high places, spread out so each cat can reach their own without competing. If the aggression keeps on, ask your vet to check the cat over, then see a qualified cat behaviourist.
Should I let my cats fight it out?
No. Separate them without using your hands, with a piece of cardboard or a cushion, and let each one calm down in their own space. Cats Protection says to separate fighting cats to avoid injury, then repeat the introduction steps more slowly.
How do you introduce two adult cats?
With the same steps, given more time. Things can move faster with a kitten, and two adults may never become friends. Battersea counts it a healthy outcome when cats simply learn to tolerate each other.
Sources and further reading
