Since cats live within a hierarchy, you have to be observant and watch for hints that territorial aggression is bubbling under the surface. The proper introduction of a new cat, or a reintroduction of current resident cats, if necessary, is the most important foundation. Make sure you have an adequate number of litter boxes in various territories around the house, as well as multiple feeding stations. Increase vertical territory and create various levels to help the cats maintain a peaceful pecking order.
If the problem is triggered by a visit to the veterinarian, put the cat in a separate room when he first comes home so he'll have a chance to groom and smell like himself again. This will help him quiet down after the ordeal and get back to normal behavior. You can also take a towel and first rub the cats who stayed home and then rub the returning cat. Don't do it the other way around because it will spread the scents from the veterinary clinic and you'll have a house full of angry cats. If you repeatedly have a problem with territorial aggression after a veterinary visit, rub the cat down with a towel before leaving and then use that towel again on the cat when he returns home.
Sometimes, no matter what you try, there are some cats who just can't coexist peacefully. In that case you have to modify the living arrangements so they can remain separated or consider rehoming a cat who might be happier in a single-cat environment.
Get personal advice from the expert now!
Reprinted from Cat vs. Cat: Keeping Peace When You Have More Than One Cat by Pam Johnson-Bennett © 2004. Permission granted by Penguin Putnam.