We studied the effect on the circulation of reducing peripheral vascular resistance by infusing sodium nitroprusside into lambs of three different age groups (subgroup A, 11-26 days, subgroup B, 32-52 days, and subgroup C, 61-88 days of age) with and without an aortopulmonary left to right shunt. Infusion of 10 micrograms/kg/min nitroprusside over 2 h decreased aortic and left atrial pressures markedly and increased heart rate, whereas systemic, pulmonary, and left to right shunt blood flows hardly changed. Within 30 min after the onset of infusion, the hemodynamic variables stabilized. Aortic and left atrial pressures were still below control levels at that time. The different flows remained the same and heart rate, after an initial fall, increased again. The pattern of hemodynamic changes was not influenced by age or the presence of an aortopulmonary left to right shunt. Based on this study, we do not advocate sodium nitroprusside administration in case of a left to right shunt with normal arterial pressure and systemic blood flow.