The influences of veno-arterial bypass (VAB) for ECMO on hemodynamics and kidneys were investigated using 15 mongrel dogs by changing the bypass flow rate to determine the optimal flow rate. The animals were divided into three groups ; 20% bypass flow rate (BFR) group (BFR=% of cardiac output) (n=5), 40% BFR group (n=5), and 60% BFR group (n=5). Systemic hemodynamics, including total circulatory blood volume, did not significantly change 1 and 2 hours after VAB commencement. Renal blood flow(RBF) did not decrease in the 20% BFR group, while RBF at 2 hours was significantly decreased in the 40% BFR group(104±13.7 vs 80±12.1ml/min ; p<0.01) and in the 60% BFR group (113±34.7 vs 84±25.6ml/min ; p<0.01). In the renin-angiotensin-aldosteron system, plasma renin abtivity (PRA) was significantly elevated at 2 hours in all groups. Angiotensin II levels showed no significant changes in the 20% BFR group, while in the 40% and 60% BFR groups the values at 2 hours were increased significantly(263±182 vs 4285±2413pg/ml;p<0.01, and 470±308 vs 1839±1408 pg/ml ; p<0.05, respectively). These results suggested that 20% BFR did not adversely influence systemic hemodynamics or RBF, but BFR more than 40% did produce unfavourable results.