Partial heart-lung bypass was performed in open-chest dogs, using the gravity arterial in-fusion technique. Arterial infusion pressure, venous suction and pressures in the left ventricle, aortic arch, and airways were phasically recorded. The mean flow in the aortic arch and the blood content of the extracorporeal circuit were continuously recorded. According to the height of the bag oxygenator above the heart level, partial heart-lung bypass resulted in a shift of blood either from the extracorporeal circuit into the animal or vice versa. When the body blood volume was increased by the bypass procedure, the cardiac output decreased only slightly. When the body blood volume was diminished by the bypass procedure, the cardiac output dropped markedly. It was found that when the systemic arterial pressure was maintained at approximately normal levels, independent of cardiac output, the change in cardiac output was proportional to the change in the body blood volume.