Administered intravenous infusions to phobic patients prior to systematic desensitization. Saline-infused subjects significantly demonstrated the predicted symptom decrease in response to systematic desensitization, whereas naloxone-infused subjects showed no change. Subject reports and psychophysiological measures of arousal indicated no discriminable awareness of the infusion condition. Suggests activation of the endogenous opiate system affects systematic desensitization's effectiveness. (Author/KS)