Why do my fixed adult male cats spray . They just started to mark their spots lately and I thought once they are fixed they don't spray ...

Neutered cats can spray as well. Ten percent of male cats neutered before 10 months of age will still spray as adults. In households with numerous cats, at least one cat will likely spray, even if all the cats are neutered.

What to try to remedy the situation:
- Neuter or spay the spraying cat.

- Identify the reasons why your cat may be spraying. For instance, if your cat is reacting to the sight of cats outside, block your cat’s view. If your cat is reacting to the scent of cats outside, possibly through a screen door or from odors on your shoes, prevent your cat from coming into contact with these scents. Keep the door closed and remove shoes outside, before entering the home.

- Discourage cats from hanging around outside your house. Motion-activated devices, such as the Critter Gitter™, the Scarecrow™, or the Scraminal™, all function to frighten outdoor cats away. The Scat Mat™ and the Sofa Saver™ can be used to keep outdoor cats away from doors and windows.

- If your cat is spraying in one or a few locations, you can make these areas less appealing, using some type of booby trap, such as Ssscat™. Ssscat™ is a motion-activated device that sprays the cat with a harmless but unpleasant aerosol. Alternatively, you can put out an “unwelcome mat” for the cat by placing foil, plastic wrap, or upside-down vinyl carpet runner where your cat sprays. Be aware that cats often just choose a new spot to spray.

- You can also try eliciting a different behavior in the sprayed locations. Place items that stimulate behaviors incompatible with spraying, such as the food dish or toys, in the spots.

- You can try placing a litter box in each location. If the cat is spraying on the wall beside the litter box, try attaching a liner on the wall and drape it down into the box. Should the cat spray there, the urine at least will drip down into the box.

- Spray Feliway™ in the areas where your cat is spraying. Feliway™ is a synthetic pheromone designed to elicit calm, friendly behavior in cats. Research supports the claim that Feliway™ reduces indoor urine spraying.

- If the spraying is due to conflict among resident cats, you should seek counsel on resolving the conflict. You may need to separate the cats or at least isolate the spraying cat until you are able to restore harmony. If this is not possible, re-homing to reduce your numbers may be the only viable solution. A spraying cat might not spray at all in a new home with fewer cats.

What not to do:
- Do not punish the cat by hitting, spanking, or slapping for spraying. Similarly, do not take the cat to the area and admonish him. This might well teach the cat to be afraid of you. The cat may actually spray more if he is stressed by the punishment.

Source: http://www.aspca.org/site/PageServer?pagename=pets_urinespraying

What I didn't know until researching this answer: Female cats can sometimes spray too!

nope.. once a sprayer, quite often, always a sprayer. you can try a pheromone disperser called 'feliway'.. talk to the vet about it.. that often helps. cats spray when they feel their territory is threatened. another good reason to keep cats indoors.