The additive effect of social and pharmacological treatments was evaluated in pairs of male mice. Ineffective and effective doses of morphine (2.5 and 5.0 mg/kg, i.p.) were tested on pain threshold in dyads of males at different times after pair formation and drug treatment. During the second hour of social interaction after reunion, saline-injected adult sibling male mice showed a decrease in nociception as measured by the tail-Hick test. Pairs of unrelated, unfamiliar control mice showed no changes in pain sensitivity during a 2-h social session. An ineffective dose of 2.5 mg/kg of morphine in nonsibling males, significantly increased tail-flick latencies in sibling pairs, before the effect of the social environment (sibling) reached statistical significance. The higher dose of morphine (5.0 mg/kg) produced analgesia in sibling as well as in nonsibling males, but the effect in the latter disappeared 60 min after drug treatment, whereas siblings were still analgesic. These results indicate that an ineffective dose of morphine, combined with the activation of the endogenous opioid system by social factors, can affect nociception. Behav Pharmacol 1998; 9:369–373 1998 Lippincott-Raven Publishers.