Recent evidence indicates that inter-male conflict in mice, but not rats, results in opiate-like analgesia in defeated animals. To test the hypothesis that differences in attack intensity/controllability may underlie this apparent species difference, male rats were exposed to non-ritualized attack from lactating conspecifics. In response to such encounters, male intruders exhibited significant elevations in tail-flick latency. Pretreatment with low (0.1-1.0 mg/kg), but not high (10 mg/kg), doses of naloxone partially antagonized the development of post-encounter analgesia and enhanced several components of male defensiveness during encounters. These results suggest that exposure to attack from lactating conspecifics activates both opioid and nonopioid analgesia mechanisms in male intruders. Correlational analyses revealed, however, that intruder analgesia was not related to the number of attacks received or the display of submissive postures. Data are discussed with reference to the possible importance of psychological factors in the activation of endogenous pain inhibitory mechanisms in rats.