Help Help cat peeing just out of liter box

We have 2 female spayed tabbys 10 and 6yrs old. The six year old has kidney disease. We give her fluid theropy at home 3 times per week. She was checked by our Vet and everything else is normal, no infectons. We have 2 liter boxes set out for them and they have been sharing for 6 yrs. About 1 month ago the 6 yr old started to urinate outside the liter box on a vinyl mat which the box sits on. She also urinates in the box. It only happens in the morning and not daily, sometimes 2- 3 days apart.THe rest of the day she uses the box. We put her and her box in the garage for a few hours, with food and water of course. She used the box fine. We brought her back in and the next morning or day later she does it again. We dont leave her in too long 4-5 hours at most because we feel bad, like we are kicking her out, I dont know if longer intervals will help. The cats have not been on the best of terms lately. the older cat has been hissing at the younger one, maybe because of the attention the younger one is getting because of the fluid theropy, I dont know. I hope there is some one who can help us we're running out of clues.

If you have had her to the vet recently and had her levels checked, then I would take a look at this thread and see if you can find your answer inside:

Inappropriate peeing

Could it be that the older cat is refusing to let the younger one use the litter box sometimes? If the older is jealous the kitty could be preventing the younger one from using it. I would try to set up the younger one a litter box of the kitty's own if that's possible. I know that my kitten Isis doesn't like it when my cat Mischief uses her litter box. She'll pounce on Mischief and it makes a great mess because Mischief just runs away whether she's done or not.

Putting the cat in the garage longer won't help. The cat most likely won't understand totally why she's in the garage.

Is the days she's missing corresponding at all with the days you are doing the kidney treatments? Something else to consider is if she is doing it just after she wakes up - perhaps she just isn't making it to the litter box.

Thank you all for your help. 3 Days and no problems. I added an uncovered liter box and another one. I have a total of 4 boxes, 3 in the one room and I added 1 box in my bedroom. She owns my bedroom so I thought I would try adding one there becaue the other cat doesnt come in there. She uses that box once daily, maybe it is doing the trick. Both cats seem to be getting along alot better. The hissing has stopped, caught them sitting together like friends again. Funny how the peeing stopped too. Next week we are going on a vaction and are planning on taking just the one cat so we can continue the fluid theropy. We have a travel camper and she should be confortable in there with us. This will be her first trip I will keep my fingers and toes crossed for when we get home that all stays well.

GREAT Job! Thanks for listening to the advice, adding litterboxes always helps if it is not health related. It does make more work for the humans, but the cats sure appreciate the opportunity of not sharing their bathroom spot. If you ever have observed a wild colony of cats, you will see that they pee in one spot then move off and deficate in another. It is genetically in them to have alternated potty stops, part of their survival mode.