Urine marking, or spraying, is one way cats communicate with other cats or people. Intact males mark more often than neutered males or females do, but neutered males and females may also urine mark. Marking often occurs when cats are not getting along. Problems may arise among cats living in the same household or when an indoor cat sees a stray through a window or door. Rarely, a cat frustrated by not being able to do something or having problems with a human to whom it is attached may spray.
Neutering intact toms may reduce or eliminate urine marking, but spaying a queen prone to spraying is less likely to stop the behavior. Separating cats that aren't getting along is one way to try to stop spraying. Keeping stray cats away from your house by strengthening fences, squirting cats with water when they come around or some other humane method may also help. Keeping your cat indoors so it can't fight with outdoor cats or blocking windows and doors so it can't see the outside cats are other options. If bad relationships have developed among your cats, careful behavior modification may be necessary.
When all else fails, cats with marking problems may respond to anti-anxiety drugs that can be prescribed by your veterinarian. The drugs are not a long-term solution and don't work in all cases. Most elimination problems are complicated and are best resolved with professional help.