Momofcats,

The first thing you need to do is take your cats to the vet to rule out a Urinary Tract Infection. Usually when cats start eliminating outside their litterbox, it means they're trying to tell you something, like a UTI.

The next thing you'll need to do, is get a black light from a pet shop and go over your entire house with it. If there are any spots where the previous cats have urinated, the black light will make the spot illuminate. Go to the pet store and get "Nature's Miracle" and follow the directions on the bottle. You can also scrub the spots with very hot water and white vinegar.

But first of all, take your cats to the vet to rule out a medical problem.

Good luck and keep us posted!