The haemodynamic changes following the administration of morphine 0.15 and 0.30 mg kg-1 i.v. were studied in 11 patients, free from known cardiac disease. All patients were acutely ill and their lungs were being ventilated mechanically. In those patients receiving 0.15 mg kg-1, the only haemodynamic change was a slight and transitory decrease in the systolic arterial pressure. In contrast, several changes were observed in patients receiving 0.30 mg kg-1: an immediate and prolonged decrease in the cardiac index was noted along with transient decreases in heart rate, stroke volume index, arterial pressure and left stroke work index. These results suggest that the haemodynamic cost of morphine 10 mg is negligible but could be significant when 20 mg has been administered and must be weighed against its beneficial effects in the critically ill patient.