Opiates and the endogenous opioids mediate maternal behavior and various forms of aggression. The present study sought to investigate the role of opiates in postpartum aggression (PPA), an intense form of agonistic behavior displayed by lactating females. Primiparous rats were screened for their PPA against adult males on day seven postpartum. They were then randomly assigned to one of four treatment groups [morphine, 5.0 mg/kg; naloxone alone, 0.5 mg/kg; morphine (5.0 mg/kg) plus naloxone (0.5 mg/kg); and saline] and tested for PPA on postpartum days eight and nine following the respective treatments. Morphine significantly lowered PPA, and naloxone antagonized the effect. Whereas the morphine plus naloxone, naloxone alone, and saline groups exhibited higher levels of PPA than that shown by the morphine group, there were no differences in PPA found among the morphine plus naloxone, naloxone alone, or saline groups. These results, in conjunction with evidence describing the state of the endogenous opioid system in the postpartum rat, suggest that some aspect of the endogenous opioid system may be involved in another form of maternal behavior, postpartum aggression.