We studied the effect on the circulation of the catecholamines isoproterenol, dopamine, and dobutamine in chronically instrumented lambs with aortopulmonary left-to-right shunts (ages 11 to 87 days) and without shunts (ages 8 to 97 days). Infusion of 0.1 microgram/kg/min isoproterenol or 10 micrograms/kg/min dobutamine markedly increased heart rate and systemic and pulmonary blood flows, while stroke volume and the left-to-right shunt flow did not change. Since pulmonary blood flow increased and the left-to-right shunt flow did not change, the left-to-right shunt fraction decreased with the infusions of isoproterenol and dobutamine. The hemodynamic changes during the infusion of isoproterenol and dobutamine occurred immediately after the start of infusion and stabilized within a few minutes. The pattern of hemodynamic changes was not influenced by the presence of an aortopulmonary left-to-right shunt or by age. Infusion of 10 micrograms/kg/min dopamine caused only small hemodynamic changes. This study shows that heart rate and systemic blood flow in the lamb are closely related. Furthermore, it demonstrates that despite an increased systemic blood flow, left-to-right shunt flow does not change after infusion of isoproterenol and dobutamine. Any decision as to which positive inotropic agent might be preferred at an early age should await experimental work concerning the effect of these agents on the myocardial oxygen demand and on the distribution of the systemic blood flow.