How to Introduce Your Adult Cat to a New One

Cat Behavior Question: How can I help my cat accept a new cat?

Photo of Coco Swatting Calypso
Coco Swats Calypso. Photo Credit: © Pat W.

Question: “How can I help my cat accept a new cat?”

Pat W. writes, “Coco seems scared of Calypso and hisses, growls or snarls when she comes over and tries to play. Coco also slinks around and is constantly on guard around Calypso. I don’t think Calypso is trying to be mean, dominant, etc. but I’m not positive it’s all just play. The problem seemed to worsen in the past week or two. I’m guessing something happened when I was at work.”

Coco, a spayed and declawed 11-year-old, was successfully treated for hyperthyroidism in January 2010. While away from home for the 10-day radioactive iodine therapy and quarantine, her 19-year-old kitty companion Raleigh died. Pat says Raleigh reacted similarly (hisses?) for the first year toward Coco who always has been very playful and fun. So a month later, Pat adopted the 18-month-old Calypso.

Pat separated the cats for three days, and rubbed towels on one cat, then the other, and let them sniff the towels. The next two weeks the cats interacted through baby gates/screen door, along with very brief supervised times together that went well—so they were given a free run of the household. But a couple weeks later, Coco’s occasional hisses turned to growls, and a few nights ago Pat heard snarls.

Amy’s Answer

The details you’ve provided, as well as steps taken to solve the issue, tell me you’re already an incredibly savvy cat person (purr-son!). You've done so much RIGHT that readers can learn great information just from your question. But even though you’ve addressed many of these things, my answer will cover the cat’s physical and emotional health, as well as traits of instinct, using the H.I.S.S. Test to help figure out what’s going on and find solutions and put this in perspective for others.


Older cats get sick quicker, and take longer to get well. Even healthy senior cats tend to have shorter tempers compared to younger felines. Most 11-year-old cats have arthritis, and hidden aches can also elevate Coco's anxiety. I’m delighted that three weeks ago Coco’s vet said she’s doing well regarding thyroid!

However, vet visits often cause turmoil when the examined kitty returns home and “smells funny” to the other felines. Just as the cats seemed to be doing well with the intros, that could have been enough to prompt Calypso to react and un-do all the good introduction work you’ve done. I wonder if the timing of that vet check corresponds to the increase in growls?


In a feral cat environment, the “newbie” cat gets his or her furry tail kicked and chased away from the valuable resources of food, shelter, and sexy kitties. Cats don’t have to fight—they can run away or keep an acceptable distance from the feline mob bosses. If the interloper cat hangs around long enough, he becomes familiar enough to the group to eventually be accepted. This doesn’t happen overnight. It can take months.

Although reproduction isn’t an issue, house cats act the same way toward intruders. This instinctive behavior kicks in at maturity, and generally speaking, the “resident cat” holds all the cards and owns the house. On top of that, the house is an artificial and very confined environment. Where a single feral cat might “own” two-square-miles or more of territory, house cats must share very limited space—and there’s nowhere to escape so they must fight.  


Stress and anxiety can make cats act with fear aggression, using hisses and growls to keep the scary thing at a safe distance. Adding a new cat into the household tops the “angst” list for our pet felines. Pretty much any change from routine can cause stress. Coco has a boatload of stressors: 1) away from home at the vet 2) sick (now recovered) 3) loss of companion (grieving) 4) intro of a new cat.

S=Symptom, Signs & Solutions

Pat says, “I separated them and now they are only getting supervised time together. I’m basically trying to reintroduce them, using screen doors again.”

Excellent! The most common cause of new cat problems is rushing the introduction process. Remember that you mention Raleigh reacted similarly “for the first year.” It can take that long for some cats to accept each other. It's been less than eight weeks...that's an eyeblink in terms of kitty timelines! And there is no guarantee that these cats will ever like each other.

I would recommend you keep a solid closed door (not a gate or screen) between the pair for at least a week. Keep new-cat-Calypso confined, while Coco gets free run of the rest of the house so Coco knows not all her territory has been invaded. Eliminating the sight of each other reduces arousal level.

Meanwhile, make the door the best place in the house by feeding them on opposite sides, offering treats, playing with them, offering catnip (get them intoxicated!). Make the association of each other’s presence a positive, fun thing. Try feeding Coco on a dish, and then using the same unwashed dish to feed Calypso and vice-versa so they start to associate their smells with favorite foods. After a week, use the gate for another week, and then no separation. But supervise--separate for at least two months when you can't watch them.

I’m glad the Bach Flower Essences helped in the past, and they may prove helpful this time as well. You mention Coco has been given Vine, Holly (aggression/suspicion/jealousy), Mimulus, and Willow, and Vervain for Calypso. My caution here would be to choose only one or two to use with Coco rather than all four. Choose those that seem the best match for the situation

The Feliway diffusers you’ve begun using should also help, but it can take ten days or longer to show effect. Since you mention most skirmishes happen in the living/dining room, be a pet detective to figure out why that location prompts angst? What prime cat resources are in that area? The best sleep spot? Window lookout? Cat tree? Coco perhaps feels protective of these valuable assets—can you create MORE of them, so there’s less for the cats to argue over? Oftentimes, cats guard doorways. Providing a kitty tunnel for a “hidden” access through this space may reduce some angst.

The key is to reduce the cat's opportunity to practice aggression, and give them time to accept each other.