The authors studied the effects of halothane on the simulated diving reflex and the Valsalva maneuver in man. Arterial blood pressure, heart rate, cardiac output by ballistocardiogram and other measurements of cardiovascular function were recorded. Diving was simulated by application of a wet cloth at 0 C to the face with simultaneous apnea. The Valsalva maneuver was effected by raising the airway pressure to 25–30 torr for 25–30 sec. Simulated diving in the conscious subject decreased heart rates as much as 29 per cent from the lowest control heart rate, compared with a 3.G per cent decrease during halothane anesthesia. Bradycardia was associated with increases in systolic and diastolic blood pressures in conscious and anesthetized subjects. The decrease in cardiac output observed in conscious subjects did not occur during anesthesia. Anesthesia apparently abolishes the parasympathetic component of the simulated diving reflex and interferes with the sympathetic component of this reflex. During phases II, III, and IV of the Valsalva maneuver greater decreases in blood pressure and increases in heart rate were observed during anesthesia. Halothane reduces or abolishes the circulatory reflex responses to diving and to the Valsalva maneuver in man.