Pulmonary shunting (Qs/Qt with the FIO2 = 1) was determined in ten young healthy non-obese patients prior to anaesthesia, 30 min after induction in the supine position and 1 h later during surgery in the prone position. Respiration was spontaneous in awake patients and was controlled following the induction of anaesthesia. Halothane was the primary anaesthetic and tubocurarine provided muscle relaxation. Mean PaCO2 was 40.7, 29.5 and 27.9 mmHg respectively in the three study periods. The shunt remained unchanged at 5.7 +/- 0.8% in the awake patients, and 6.0 +/- 1.1% during anaesthesia in the supine position, and 5.2 +/- 1.2% during anesthesia and surgery in the prone position. It is concluded that in this population the prone position does not alter the magnitude of the pulmonary shunt even in those patients who develop shunts of over 10% after the induction of anaesthesia.