OBJECTIVE This study was undertaken to determine the optimum ventricular pacing rate at which the optimal cardiac function can be attained in fetal lambs with complete atrioventricular block. METHODS Complete atrioventricular block was created by cryosurgery in 12 fetal lambs, and the ventricle was paced randomly at rates of 60, 90, 120, 150, 180, 210, 240, and 270 beats/min. The aortic pressure, central venous pressure, and right ventricular cardiac output were measured before cryosurgery and during ventricular pacing. RESULTS Complete atrioventricular block was created in 9 of 12 fetal lambs. Systolic aortic pressure significantly decreased at pacing rates of 60, 90 and 240 beats/min (P <.01), and central venous pressure was increased significantly at a pacing rate of 60 beats/min (P <.01) compared with the control. Right ventricular cardiac output significantly decreased at pacing rates of 60, 90, 120, 180, 210, 240, and 270 beats/min (P <.01), although no significant difference was found at the pacing rate of 150 beats/min compared with control. CONCLUSIONS Ventricular pacing at a rate of 150 beats/min gave the highest aortic pressure, the lowest central venous pressure, and the highest right ventricular cardiac output in fetal lambs with complete atrioventricular block.