The hemodynamic effects of left atrial administration of protamine for heparin reversal were compared with the peripheral venous route. One hundred patients, undergoing cardiac surgery, using cardiopulmonary bypass (CPB) at Siriraj hospital were randomly allocated into two equal groups of fifty. The preoperative and operative characteristics of the two groups were comparable. After the termination of CPB, protamine sulfate was administered over 3 minutes via the left atrium in group I, and via the peripheral vein in group II. The hemodynamics were measured before and 5, 15 and 30 minutes after protamine administration. There were no statistically significant differences in heart rate (HR), arterial blood pressure (BP), and central venous pressure (CVP) between the groups. After administrating protamine and adequate maintaining the preload in both groups of patients within the normal range, the HR did not change from immediate post CBP control values. The arterial BP was not changed for 5 minutes, thereafter, significantly elevated toward the preoperative baseline values. This data indicates that the left atrial injection of protamine does not provide any hemodynamic advantages over the peripheral venous administration.