Intravenous morphine infusions have been administered to 12 critically-ill patients during controlled ventilation. Acute oliguric renal failure was present in 4 patients, who were treated with a combination of haemofiltration and haemodialysis. Severity of physiological disturbance was assessed using a modified APACHE Score, level of sedation by a linear-analogue scale, and blood morphine levels by high-pressure liquid chromatography. Morphine clearance was impaired in renal failure, and was dependent on haemofiltration volumes; accumulation of morphine did not occur during this form of treatment. Conscious level was clearly more closely related to the degree of physiological disturbance than blood morphine levels; and for a given blood morphine level, depression of consciousness was more pronounced the greater the degree of physiological disturbance. Use of a physiological sickness score may help to clarify some of the factors influencing cerebral function during critical illness. Careful clinical monitoring of level of sedation is important in patients with oliguric renal failure receiving morphine, and haemofiltration appears to reduce the risk of morphine accumulation in these patients.