In a double-blind prospective study the effects of low-dose intramuscular ketamine (1 mg/kg) were compared to pethidine (1 mg/kg) in the treatment of pain after pulmonary surgery. Thirty patients were admitted to the study and postoperatively randomized to either a ketamine or a pethidine group. The analgesic effect was evaluated using a scale ranging from 0 to 10, where 0 denoted no pain and 10 severe pain. We did not find any significant difference between the analgesic effect of ketamine and pethidine; however, the duration of action of ketamine appeared to be slightly longer. Throughout the study PaCO2 was significantly lower in the ketamine group. PaO2 increased through the study in both groups and was significantly higher after 2 h. Heart rates increased significantly only in the pethidine group. Mean arterial pressures remained unchanged and the respiratory frequencies were similar in the two groups. The incidence of adverse reactions was low and not significantly different between the groups. The findings indicate that low-dose intramuscular ketamine is a potent analgesic for postoperative analgesia following thoracic surgery and that it has no respiratory depressive effect.