Because of its analgesic and amnesic effects, ketamine has potential advantages as a primary agent for patients undergoing open-heart surgery. However, the undesirable positive inotropic and chronotropic effects associated with ketamine have deterred its use. Diazepam pretreatment appears to block these unwanted effects. Sixteen randomly selected patients were given a combination of diazepam, 0.4 mg/kg, followed by ketamine, 2 mg/kg, and nitrous oxide, 50%. The authors compared the circulatory responses to induction, intubation, and operation with those obtained in a matched group of patients who received morphine, 3 mg/kg, and nitrous oxide, 50%. All patients underwent mitral- or aortic-valve replacement. Circulatory responses were not significantly different between the two groups. In both groups, heart rate, mean arterial blood pressure, and rate-pressure product increased significantly with intubation of the trachea, incision of the skin, and sternotomy. The incidences of hypertension, hypotension, and arrhythmias, and the need for inotropic drugs were not significantly different between the two groups. No intraoperative awareness occurred in either group. It is concluded that diazepam-ketamine anesthesia is a satisfactory alternative to morphine anesthesia for patient undergoing heart-valve replacement.