The effects of halothane anesthesia on cardiopulmonary (low pressure) baroreflex control of peripheral resistance were studied in 10 ASA class I young men. Graded (-5,-7.5, −10, −12.5 mmHg) lower body negative pressure (LBNP) was used to produce progressive decreases in thoracic blood volume and central venous pressure. These stimuli activate reflexes from cardiopulmonary baroreceptors. Volunteers were studied while awake and during 1 MAC (0.75%) and 1.25 MAC (0.93%) halothane anesthesia. Hetastarch (6%) in 0.9% normal saline was infused into patients before baseline recordings were initiated. Blood pressure, stroke volume, cardiac output, and systemic and forearm vascular resistance decreased and forearm blood flow increased during halothane anesthesia. In awake subjects, LBNP did not alter heart rate or blood pressure, but stroke volume and cardiac output decreased. Blood pressure was maintained by cardiopulmonary baroreflex-mediated increases in peripheral resistance. In anesthetized subjects, decreases in stroke volume and cardiac output during LBNP were similar to awake responses, however, hypotension occurred because reflex resistance increases were markedly attenuated. The authors conclude that halothane anesthesia blunts cardiopulmonary baroreflex resistance responses provoked by mild decreases in thoracic blood volume in humans.