It is important to be sure he is spraying as opposed to urinating outside of the box. If he is urinating outside the box, he needs to be checked out by a veterinarian. He could have urinary tract infection, renal problems, crystals in urine, diabetes, etc. A general checkup with blood and urine analysis will likely reveal something.

If you actually see him marking, he could be triggered by a change in the household, or an unaltered male cat that is passing by your place and marking outside. Sometimes just a neighborhood cat sitting on the fence giving him a cocky 'look' is all that it takes.

A male cat that begins marking before he is neutered has potential to mark after neutering if he is unhappy.
Source(s):
http://www.catinfo.org/

Permalink | Report