Forty-nine patients undergoing elective total hip replacement received either morphine or meptazinol for postoperative analgesia from a patient-controlled analgesia apparatus. Ventilatory rate and volume and arterial oxyhaemoglobin saturation were recorded continuously for the first 24 h following surgery. Episodic hypoxaemia was seen in both groups, associated with disturbances in ventilatory pattern. There was no significant difference in the incidence or severity of observed hypoxia between the groups, or with respect to the class of ventilatory disturbance. Mean linear analogue scores for pain and nausea were significantly (P less than 0.05) greater in the meptazinol group than in the morphine group 8 h after operation, but did not differ significantly at any other time. The mean number of demands for analgesic drugs was similar in the two groups. The meptazinol group had a greater requirement for anti-emetic drugs than the morphine group (P less than 0.05). It was concluded that meptazinol and morphine in equianalgesic doses had similar effects on ventilation in the postoperative period.