they have actually found a 'catnip' gene in cats which predispose them to a fondness for the stuff.
we have 4 cats - one ignores the catnip completely, another will sniff at it and roll in it, a third will eat a bit of it -- but the 4th cat gets all glassy-eyed and dazed just at the slightest wiff and then eats it all up...
http://chemistry.about.com/library/weekly/aa103001a.htmCatnip
Catnip, Nepeta cataria, is a member of the mint or Labiatae family. This perennial herb is sometimes known as catnep, catrup, catwort, cataria, or catmint (although there are other plants that also go by these common names). Catnip is indigenous from the eastern Mediterranean region to the eastern Himalayas, but is naturalized over much of North America and is easily grown in most gardens. The generic name Nepeta is said to have been derived from the Italian town Nepete, where catnip was once cultivated. For centuries humans have grown catnip for humans, but the herb is best known for its action on cats.
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Domestic and many wild cats (including cougars, bobcats, lions, and lynx) respond to the nepetalactone in catnip. However, not all cats react to catnip.
The behavior is inherited as an autosomal dominant gene; 10-30% of domestic cats in a population may be unresponsive to nepetalactone. Kittens will not show the behavior until they are at least 6-8 weeks old. In fact, catnip produces an avoidance response in young kittens. The catnip response usually develops by the time a kitten is 3 months old. When cats smell catnip they exhibit a range of behaviors that may include sniffing, licking and chewing the plant, head shaking, chin and cheek rubbing, head rolling, and body rubbing. This psychosexual reaction lasts for 5-15 minutes and cannot be evoked again for an hour or more after exposure. Cats that react to nepetalactone differ in their individual responses.