The purpose of this project was to investigate the effects of high rates of dopamine infusion on cardiac output and regional blood flow in the Iamb. We studied eight unanesthetized newborn lambs (mean age 7 ± 2 days) during a 15-inin baseline period and while infusing dopamine at 5-, 20-, 80-, and 160 μg/kg/min. We measured cardiac output and mean aortic, pulmonary arterial and left atrial pressures, and organ blood flow using radionuclidelabeled microspheres at each rate of dopamine infusion. Cardiac output increased significantly with increasing rates of infusion up to 80 μg/kg/min but decreased at 160 μg/ kg/ min. Aortic, pulmonary arterial, and left atrial pressures increased at rates of infusion above 5 μg/ kg/ min. Blood flow to all organs was unchanged at the 5 μg/ kg/ min rate of infusion of dopamine while blood flow to the brain and heart increased at the 80 μg/ kg/ min rate of infusion and blood flow to the gut and kidney decreased. We conclude that dopamine is an effective inotropic agent in the newborn lamb but that an inotropic:afterload mismatch exists at high infusion rates. Despite an increase in cardiac output at low rates of infusion, dopamine did not selectively vasodilate the vascular bed of any organs tested. Furthermore, at high rates of infusion dopamine actually impaired blood flow to the gut and kidney.