The effects of controlled vasodilation on blood flow and oxygen consumption above and below the aortic occlusion during crossclamping of the thoracic aorta were examined in 16 mongrel dogs anesthetized with halothane. Blood flow in the inferior vena cava was measured with an electromagnetic cannulating flow probe, and cardiac output was measured by thermodilution. The animals were divided into two groups. In the control group the thoracic aorta was crossclamped at the diaphragmatic level for 30 minutes. In the sodium nitroprusside group the aorta was also crossclamped for 30 minutes, but an infusion of sodium nitroprusside was initiated after 10 minutes of occlusion to decrease systemic blood pressure. Measurements were made before and at 10, 20, and 30 minutes after aortic crossclamping, and at 10 minutes after aortic unclamping. Crossclamping of the thoracic aorta was associated with marked decreases in blood flow and oxygen consumption in organs and tissues below the aortic occlusion in both groups. Above the occlusion, blood flow increased but oxygen uptake decreased. Sodium nitroprusside increased cardiac output and blood flow above the aortic occlusion even more than crossclamping alone while it decreased blood flow and oxygen consumption below the crossclamp.