Systemic hypotension is commonly observed in association with the administration of protamine after cardiopulmonary bypass. To elucidate the circulatory changes and arterial blood gas changes of protamine sulfate administration, we studied 27 patients following cardiopulmonary bypass. In the intraaortic protamine administration group (N=5) , the mean arterial pressure decreased significantly (p<0.05) in 5 minutes during protamine infusion and after protamine, infusion respectively. Heart rate, central venous pressure and blood gas analysis did not change significantly. In intravenous protamine administration group (N=22), the arterial oxygen tension increased significantly(p<0.05 %amp; p<0.01) in 10 minutes during protamine infusion and after protamine infusion respectively. There were no special changes when the more than 0.3mg/kg/min rate of protamine was injected but the mean arterial pressure decreased significantly(p<0.05) in 5 minutes during the protamine infusion. Mean arterial pressure, heart rate, central venous pressure and blood gas analysis did not change in the less than 0.3mg/kg/min rate of protamine was injected. There were no circulatory changes in the less than one hour CPB time group but the arterial oxygen tension increased significantly in 5 minutes and 10 minutes during the protamine infusion and after the protamine infusion. But in the more than one hour CBP time group, the mean arterial pressure decreased significantly in 10 minutes during the protamine infusion and after protamine infusion.