According to the opioid hypothesis of social attachment opiate receptor blockade should increase the need for social contact. Yet naloxone reduces play fighting [11], a major form of social interaction in young rats. This observation might be reconciled with the opioid attachment hypothesis if it could be shown that naloxone produced compensatory increases in other social activities or if naloxone shortened play fighting bouts without reducing their frequency. In the present experiment naloxone reduced play fighting in a dose-dependent fashion. However, the frequency of play bouts was reduced, their duration unchanged and no compensatory increase in social sniffing and grooming was observed. In addition, naloxone inhibited rearing almost as potently as it affected play fighting.