The effects of treating captive talapoin monkeys acutely (twice daily for 7 days) with naltrexone hydrochloride (0.25 mg 0.5 mg and 1 mg/kg intramuscular injections twice daily), naloxone hydrochloride (0.5 mg/kg IM twice daily) and sulpiride (1.5 mg/kg IM twice daily) was studied in social pairs and singly caged animals. The behaviour of social pairs and endocrine changes in all treated monkeys were monitored before, during and after withdrawal of the course of drug treatment. Naltrexone and naloxone, but not sulpiride, significant increased grooming and grooming invitations while aggressive behaviour, self grooming, scratching and general locomotor activity were unaffected. There was an overall increase in LH, testosterone and cortisol in plasma samples taken 60 mins after opiate receptor blockade. Prolactin was unchanged but increased dramatically in animals treated with sulpiride. No significant endocrine changes were observed to precede the increased grooming behaviour which opiate receptor blockade induced. The behavioural changes reported for this primate support the view that positive affect arising from social bonds may be mediated by cerebral endorphin containing systems.