Question: Why do cats spray urine?

Although I've heard friends complain about their cats spraying urine, I've never experienced it first-hand before today. I have two cats, both boys: Bob, a three year old and Willie, a 5-month-old kitten I adopted three weeks ago. They seemed to be getting along well, however, today, when I was playing with Billy on my bed, I saw Bob watching us with his tail sort of twitching.

He then proceeded to walk over to my bedroom door, turned his back on it, and stood there with his tail straight up and kind of quivering. I walked over to see what he was doing and saw very strong smelling urine running down the door.

Answer:

You've given a perfect description of the typical scenario where a cat will spray urine.

I'll answer your main question first:

Why Cats Spray Urine

Cats stake out their territory outdoors by spraying urine on trees, fences, vehicle wheels, and other inanimate objects. This warns other cats to stay out of that territory or be prepared to fight. Quite logically, this behavior is called "territorial marking." A cat is very efficient at keeping his territory secure. He will visit each location regularly, sniff at it to make sure only his own scent is present, and usually will enforce the message by spraying again.
Cats do much of the same indoors, for various reasons, usually involving stress caused by various factors. Cats are creatures of habit and respond to changes in their surroundings negatively, often by spraying urine.

Here are just a few examples:

• New Cat in the Family
  The introduction of a new cat may trigger a urine spraying response, particularly by an alpha cat.
• Move to Another Home
  Cats respond to most changes with stress. A move to another home will trigger the need to set new territorial boundaries.
• Severe Illness or Death of a Family Member
  I've seen urine spraying caused both by the death of a family pet and by a human family member.

Sprayed Cat Urine Has a Distinctive Odor

Cats have scent glands in several areas of their bodies, which disperse pheromones. The territorial marking urine contains an "unfriendly" pheromone, which accounts for its extremely pungent odor, unlike the "friendly" pheromone cats mark with their facial scent glands. According to Pam Bennet-Johnson, writing for the About.com Veterinary Medicine site,

Then we come to those pheromones at the back end. Oh boy! Pheromones released during spraying are related to stress and excitement. There's nothing calm about those pheromones. When a cat sprays it's done under stressful circumstances.

"Whole" (Un-neutered) Cats Are More Likely to Spray Urine
Although some neutered male cats will spay, this usually happens only if they were neutered after reaching sexual maturity. This emphasizes the importance of early spay and neuter for all pet cats.

Thorough Cleaning of the Sprayed Urine is Critical

Unless you remove all traces of urine spray, a cat will return again and again to spray the same surface. The best way to accomplish this is through the use of an enzymatic product, which will neutralize the bacteria which contributes to the odor.

• Before You Buy Urine Odor Removers
• Top Picks: Cats Urine Odor Removal Products
• How to remove cat urine odor from laundry

Friendly Pheromones to the Rescue

It is interesting that a cat will not spray urine within it's own "friendly pheromone marked" territory. Several years ago, a veterinarian-designed product was produced, which mimics the friendly facial pheromones a cat uses for this type of marking.
Cat Urine Spraying Prevention Products

Your older cat's urine spraying is apparently the result of jealousy over the young kitten. If you will follow the instructions I've provided, you'll be on your way to a peaceful, clean-smelling household again. Your next step will be to help resolve Bob's feelings of insecurity over Willie. You can best accomplish this by setting up pleasant activities together, so Bob can associate Willie with pleasure, rather than view him as a threat. Try to involve them in mutual play with a wand toy. Sit on a sofa or your bed with one cat on each side. Stroke and cuddle with each of them equally. Feed them special treats together, making sure that each cat has his own fair share of treats as well as your attention. With diligence and love on your part, your two cats should form a bonded family unit in due time.