I just read this in the vet's newsletter and thought it was curious....

Catnip is a member of the mint family, which has about 250 species. The essential oil in catnip, nepetalactone, has a powerful effect on cats that are sensitive to it, turning some of even the most sedentary couch potatoes into a flipped-out ball of ecstasy. The most intense catnip experience is an olfactory one—your cat smells the herb and promptly goes nuts. Researchers aren't sure what the neurological explanation is, but it's thought that catnip mimics feline "happy" pheromones and stimulates the receptors in the brain that respond to those pheromones. When eaten, however, catnip seems to have the opposite effect: the cat may become very mellow.

Most cats react to catnip by rolling, flipping, rubbing, and eventually zoning out. They may meow or growl at the same time. Other cats become hyperactive, running around like their tails are on fire, and some get downright aggressive, especially if you approach them. They must protect their space and their toys! Usually these sessions last about 10 minutes, after which your cat loses interest. It may take as long as two hours for him to "reset" and become susceptible to catnip again.

Not all cats fall for catnip. It's an inherited sensitivity, and if your cat didn't get that gene, he simply won't care about catnip. An estimated 50 percent of cats don't respond. The trait doesn't emerge until a cat is between three and six months old; until then, a kitten will not have a response.