We investigated short-term hemodynamic effects of dopamine and dobutamine in eight patients with acute hypoxemic respiratory failure. We tested the hypothesis that for a similar increase in cardiac output, left ventricular filling pressure (pulmonary capillary wedge pressure [PCWP]) would increase with dopamine and decrease with dobutamine. Dopamine increased cardiac output (p less than 0.05), stroke volume (p less than 0.05), and PCWP (p less than 0.01). Cardiac output increased almost 20 percent when PCWP increased 50 percent with dopamine. In contrast, despite a mean 30 percent increase in cardiac output with dobutamine (p less than 0.01), PCWP decreased. In six of these patients, left ventricular end-diastolic volumes and end-systolic volumes were measured using scintigraphic techniques. In all patients, end-diastolic volume increased with dopamine (p less than 0.05); and in four of six, end-systolic volume increased. In contrast, with dobutamine, in five of six patients, end-diastolic volume decreased; and in all six patients, end-systolic volume decreased. There was a small increase in intrapulmonary shunt with both drugs. We conclude that if an inotropic agent is required to increase cardiac output in patients with acute hypoxemic respiratory failure, dobutamine is probably preferred over dopamine.