A comparison of the respiratory depressant effects of intravenous injection of pentazocine (30 mg) or pethidine (30 mg) was undertaken in anaesthetized patients using a spirometric technique, after each patient had received a pre-anaesthetic medication of either pentazocine (40 mg) or pethidine (100 mg) by intramuscular injection. Results from four groups of patients were compared. Pentazocine given intravenously after either pethidine or pentazocine premedication, produced essentially the same degree of respiratory depression. Pethidine following premedication with pentazocine also produced an approximately similar degree of depression, but pethidine after pethidine premedication produced a cumulative and significantly greater degree of respiratory depression than that seen in the other three treatment groups.