Introducing Cats to Cats HOMEWARD TRAILS RESOURCE LIBRARY: CATS!
Help settle the new cat in a small room with litterbox, food, water, toys, and a safe place to hide (such as a cat carrier with a towel inside).
Let the new kitty become comfortable there for 3 or 4 days. Do not allow the cats to interact during this time. It is fine for the resident cat to be sniffing under the door and investigating.
Switch bedding and other items that have the scent of each of the cats on them. This way each cat can become used to the scent of the other without meeting face to face.
Spend quality time with each cat on either side of the door – petting, playing, relaxing. Again, they will be aware of each other, in a non-stressful situation. This is often reassuring to both of them.
If at any point the hissing is intense, or either cat is growling, continue to keep them separate for as long as it takes the upset cat(s) to settle down. Other signs of stress are: not eating, not using the litterbox appropriately, overgrooming, etc. (If these symptoms are apparent in your resident cat, please call your veterinarian. If the adopted cat shows these signs, please call The SF/SPCA.) This may mean that the separation needs to last a week or more. If the cats’ interaction is more intense than you feel is normal, please contact your cat’s Adoption Coordinator.
If there is no intense hissing from either cat (i.e. loud hissing with wide open mouth and teeth showing, or multiple hisses), prop the door open just about an inch to allow the cats to view each other without being able to make contact. Leave the door like this for a few days. Watch their interactions; if no serious hissing or aggression is noted, then it’s time for the next step.