Ever since another cat was adopted, our first cat has taken to marking everywhere on the walls and carpet. Is there any way to correct this problem?

Feline behaviorist Pam Johnson-Bennett says:
Every time you add a cat, all the cats in the family have to renegotiate territorial rights. Since cats are very territorial, it can get to be stressful because the cat numbers increase but the size of the house doesn't.

Your cat feels as if he doesn't have a space of his own. Perhaps the recently added cat has taken over some of his favorite spaces.

Make sure that there are enough litter boxes in various locations. Ideally, there should be as many litter boxes as cats.

There is a spray available from your vet that helps with urine-marking behavior. When sprayed on prominent objects in the house, it helps cats feel as if they have already identified it as theirs. It can have quite a calming effect on cats.