BACKGROUND: There has been no report about the effects of blood pressure (BP) on the change of blood flow (BF) to major organs when pump flow is maintained during cardiopulmonary bypass (CPB). We evaluated the changes of the BF and oxygen consumption of major organs when BP was controlled by vasopressors or vasodilators during CPB. METHODS: Carotid, femoral, hepatic and renal arteries and veins were exposed and arteries were cannulated for pressure monitoring, except the hepatic artery and arteries on the opposite side were exposed for the measurement of BF in 7 dogs. Temperature was lowered to 30oC after initiation of CPB and phenylephrine or sodium nitroprusside was infused to increase or decrease BP about 30% under the same pump flow. BP and BF were measured before CPB, before the infusion of drugs and when BP was changed by vasoactive drugs. Blood gas analyses were performed from the artery and each vein while the BF was measured. RESULTS: The change of BP didn't affect carotid and renal BF. However, hepatic BF decreased about 50% when BP was reduced and femoral BF changed in the opposite way of BP change. Oxygen consumption of each organ wasn't influenced by BP. CONCLSIONS: When pump flow was constantly maintained, changes in BP redistributed BF to major organs but didn't affect oxygen consumption. The brain and kidney have the ability of autoregulation of BF unlike the liver or legs. Hepatic BF was dependent on perfusion pressure and a decrease in BP by vasodilators during CPB may be not good for the liver.