Exposure is a rapid and effective treatment for simple phobias. This study tested the assumption that endorphin release may be involved in exposure to a feared situation. Thirty spider-phobic Ss underwent exposure to 17 phobic-related, graded performance tasks. Half the Ss were randomly assigned to naltrexone, an opioid antagonist, and half to a placebo. Measures of heart rate, blood pressure, self-efficacy, anxiety, and cognitions were obtained during treatment. Six of the 15 Ss in the naltrexone group dropped out after the 10th step in the treatment compared with 1 of the 15 Ss in the placebo group, chi 2(1, N = 30) = 4.7, p = .03. The naltrexone group took significantly longer to complete the first 10 steps (the last step that included all Ss) compared with the placebo group, F(9, 252) = 2.17, p = .024. Maximum heart rate and anxiety were significantly greater at Step 10 in the naltrexone group, but no differences were found for self-efficacy or cognitions. The study provides further evidence that the endogenous opioid system may be involved in the process of exposure.