An intravenous infusion of norepinephrine, 2 microgram/kg/min, for 5 hr in anesthetized, mechanically ventilated dogs produced a statistically significant increase in the pulmonary shunt, the cardiac output, and the mean pulmonary artery pressure, but no significant change in the pulmonary wedge pressure and the pulmonary vascular resistance except at 5 min. These hemodynamic changes are qualitatively similar to those produced by the same dose of epinephrine infused for the same time period. However, there were differences in the magnitude of the hemodynamic changes between norepinephrine and epinephrine, as might on the basis of their different pulmonary pharmacologic effects, but the changes were not statistically significant at all times, and more studies are necessary to determine if the differences are real. These studies add support to the thesis that adrenergic stimulation plays an important role in the pathogenesis of the pulmonary insufficiency associated with shock.