The effects of oral administration of buprenorphine ('buprenorphine jello'), a partial mu opioid agonist, oral naltrexone, a mu antagonist and morphine, a mu agonist, were investigated in rats following laparotomy. Food and water consumption and body weight were reduced in rats that underwent surgery. Rats undergoing anaesthesia alone showed only a small reduction in water consumption. Administration of oral buprenorphine (0.5 mg/kg in flavoured gelatin) decreased the effects of surgery on body weight and water intake when compared to untreated (vehicle alone) controls. The magnitude of this beneficial effect was similar to that seen in previous studies using subcutaneous administration of buprenorphine. The fall in body weight and food and water intake following surgery was similar in the groups which received morphine and the control group which received vehicle (jelly). Neither the magnitude of the fall in body weight, and food and water intake, nor the behavioural scores differed between naltrexone and control (vehicle alone) rats following surgery. This suggests that the beneficial effects of partial agonist analgesics are mediated by a reduction in pain rather than by antagonism of endogenous opioids. Both anaesthesia and surgery caused changes in behaviour, but the major effects of buprenorphine in normal (unoperated) rats severely limited the value of behavioural parameters as a means of assessing possible beneficial effects of analgesic administration.