The relative vasodilating properties of isoproterenol, dopamine, and dobutomine on hypoxic pulmonary vasoconstriction were examined in isolated pig lungs, ventilated and perfused in situ. The flow-resistant properties of the pulmonary circulation were assessed by measuring the relationship between pulmonary arterial pressure (PAP) and flow (Q). Hypoxia (a decrease in inspired O2tension from 200 torr to 50 torr) caused a rightward shift and decreased in the slope of this relationship. This increased PAP an average of 18.8 torr when measured at a blood flow of 1 L/min. Isoproterenol and dobutamine inhibited hypoxic pulmonary vasoconstriction, whereas dopamine had no effect. Because of its relative freedom from adverse side-effects in clinical trials, dobutamine should be considered in patients with hypoxic pulmonary vasoconstriction and right heart failure.