The respiratory depressant actions of pethidine and tilidine during anaesthesia were compared in 18 surgical patients anaesthetized with N2O + O2 after thiopental induction. Five minutes after thiopental, 0.5 mg/kg pethidine or 1.5 mg/kg tilidine were each given intravenously to six patients, the remaining six patients serving as controls. Minute ventilation, respiratory rate, end-tidal CO2 and PCO2 from arterialized venous blood were measured up to 30 min. Pethidine caused the following maximal changes: V -0.98 +/- 0.24 (s.e. mean) 1/min, rate -5.5 +/- 0.7/min, CO2ET + 0.7 +/- 0.1 vol % and PCO2 + 5.7 +/- 1.1 mm Hg. These changes occurred within 10 min of the injection.