Recent studies have shown that intruder mice display qualitatively different forms of analgesia in response to social encounters with resident conspecifics. An acute (non-opioid) reaction is evident when display of defeat is used as test criterion whereas a much longer-lasting (opioid) response is observed when this criterion is exceeded. Neither reaction can be readily attributed to the effects of physical stimulation per se. Two studies were conducted to determine the critical stimulus leading to a 'switchover' from non-opioid to opioid analgesia in intruder mice. The pattern of results indicates that temporal factors per se are unimportant but that extended exposure to attack is essential to the development of an enduring (opioid-typical) analgesia. It is argued that this finding is consistent with the uncontrollability hypothesis which asserts that opioid analgesia is a consequence of a failure in behavioural coping strategies.