The effects of three graded mid-line abdominal operations were investigated in rats. All of the surgical procedures caused a significant reduction in food and water consumption, body weight and locomotor activity. Animals which had the skin incision alone showed significantly less depression of food and water consumption and body weight than groups which underwent laparotomy. The detrimental effects on water consumption and body weight could be significantly reduced by the administration of the opioid analgesic buprenorphine (TEMGESIC, Reckitt & Colman) (0.05 mg kg-1, s.c.). The stepped response to graded surgery, and the reduction of the depressant effects of surgery on food and water consumption by buprenorphine, suggest that some of these changes may be related to the presence of pain after an operation.