Effects of prostaglandin E1 (PGE1) and phenoxybenzamine (POB) on the hemodynamics during cardiopulmonary bypass (CPB) were studied in 30 infants and children. Patients were grouped into three; PGE1 was given to ten patients, POB to another ten, and the other ten patients served as the control. Vasodilating drugs were withheld. PGE1 was infused at 0.01 to 0.02 microgram/kg/min during CPB, and POB at 1.0 mg/kg within the initial 10 minutes of bypass. There was a significant drop in arterial and venous pressure at the time of initiation of bypass in both the PGE1 and POB groups. In the PGE1 group in particular, such a stable hemodynamic condition of over 60 mm Hg in mean arterial pressure, 7.5 to 12.5 cmH2O in central venous pressure, 1300 to 1700 dynes . sec . cm -5 in systemic vascular resistance was maintained throughout CPB, as compared with the other two groups. PGE1 contributed to an adequate diuresis and the preservation of platelets. Our findings indicate that PGE1 has potential clinical advantages for application during CPB.