Subjects were randomly assigned to either a morphone condition or a placebo condition. They were informed that they would be competing with other subjects in the adjoining room. Shock "unpleasantness" thresholds for both subject and opponent were determined by administering a series of increasing shock intensities. Each subject competed with the opponent for a series of 21 trials and completed a post-task questionnaire following the reaction time task. Aggression was defined as the magnitude of shock that the subject selected for his opponent to receive. Results showed that subjects in the morphine condition were more willing to initiate attacks against their opponent than subjects in the placebo condition. Further, subjects in the morphine condition reacted more aggressively than subjects in the placebo condition at all levels of provocation. Study findings suggest that the traditional view of opiates reducing aggressive behavior requires re-examination. A possible explanation for enhanced aggression in the morphine condition is offered that involves the analgesic properties of morphine. Another potential hypothesis, that morphine-induced aggression is mediated by arousal changes, is not supported. 20 references and 1 table