BACKGROUND: It has been known that radial arterial pressure sometimes underestimates central pressure after cardiopulmonary bypass (CPB) and status of circulatory volume, vascular tone, hemodilution or rewarming were explained as the major causes of this phenomenon. We evaluated the effects of these variables on blood pressure and flow in radial and femoral arteries in dogs undergoing CPB. METHODS: Both radial and femoral arteries were cannulated for pressure monitoring and the opposite side arteries were exposed for the measurement of blood flow in 9 dogs. Radial arterial pressure (RBP), femoral arterial pressure (FBP), aortic pressure (ABP), radial blood flow (RBF), and femoral blood flow (FBF) were measured before and just after the initiation of CPB. All values were also recorded when pump flow or ABP were changed during hypothermic CPB and after rewarming. RESULTS: Unlike patients with cardiac disease, RBP, FBP and ABP were almost the same and changes in pump flow and vasoactive drugs didn't make any clinically significant pressure difference between RBP and ABP during hypothermic CPB or after rewarming in the normal condition of dogs. RBF was constantly maintained when the ABP increased with phenylephrine or decreased with sodium nitroprusside (SNP). However, FBF markedly increased during SNP infusion. CONCLUSIONS: Cardiac output, vasoactive drugs and even CPB didn't exert any effect on RBP and FBP in dogs with a normal cardiovascular system. Cardiovascular abnormalities and vascular change or hemodynamic instability itself may induce a large discrepancy between RBP and ABP in cardiac patients undergoing CPB.