Fear of spiders may be caused by a lack of natural tranquilizers in the brain, says a Stanford University research team that intends to find out for sure through hands-on tests of spider-phobic people.

Volunteers who are scared of spiders will come to the California research institute for what might be called "spider therapy." They will be exposed to a spider in a series of 14 steps, beginning with merely seeing one at a distance in a closed glass jar, and progressing, if that's the word, to actually touching the spider.

Some volunteers will be given naltrexone, a drug that blocks production of natural tranquilizers called endorphins. The response to therapy of these volunteers, including the levels of endorphins in their brains, will be compared with the response of those who don't receive the drug. The tests will be conducted by Dr. K. Gunnar Gotestam and his colleagues at Stanford's Department of Psychiatry and Behavioral Sciences.

Gotestam says the sight of a spider may trigger an instinctive "fight or flight" response that thousands of years ago helped protect humans. But, he theorizes, many people's brains manufacture and release endorphins, natural painkillers that temper the fear and enable them to overcome their fear of spiders.

Failure to release such endorphins, Gotestam guesses, may cause excessive fear of spiders, which in extreme cases can cause people to become reclusive.

Researchers said volunteers may drop out of the study at any time.