• We studied the short-term hemodynamic effects of captopril in ten infants with congestive heart failure secondary to large left-to-right shunts who were refractory to routine medical management with digoxin and diuretics. During cardiac catheterization, captopril (0.5 to 1.0 mg/kg) was administered by nasogastric tube. For the entire group, mean systemic blood flow, pulmonary blood flow, and the pulmonary-to-systemic blood flow ratio did not change significantly after captopril administration. However, in the seven patients in whom baseline systemic vascular resistance was greater than 20 U/m2, captopril decreased the pulmonary-to-systemic blood flow ratio. In contrast, in the three patients in whom baseline systemic vascular resistance was less than 20 U/m2, captopril increased the pulmonary-to-systemic blood flow ratio. We conclude that captopril acutely decreases the pulmonary-to-systemic blood flow ratio in infants with large left-to-right shunts who have elevated systemic vascular resistance. Renal function must be monitored closely when using captopril.