This study tested the hypothesis that endogenous opioids are involved in the extinction of phobic fear through exposure in vivo. Forty-eight spider phobics participated in a 2-hr therapist-directed exposure in vivo treatment. Sixteen Ss were assigned to placebo, 16 to a low dose of naltrexone, and 16 to a high dose of naltrexone. Before intervention, after treatment, and at a 1-wk follow-up test, self-report, physiological, and behavioral measures of phobic fear were completed. At 1-wk follow-up, naltrexone was significantly related, in a dose-dependent way, to a greater relapse on avoidance measures but not on emotional, cognitive, and physiological measures. Endogenous opioids may be specifically involved in the extinction of avoidance behavior but not in the extinction of all aspects of phobic fear.