This study was designed to evaluate the effects of dopamine and dobutamine on hemodynamics and plasma catecholamine levels during experimental lactic acid acidosis in dogs. During the normal acid-base state (pH 7.4, PCO2 40 mm Hg), cardiac output and stroke volume were significantly increased and systemic vascular resistance was decreased by the infusion of dopamine or dobutamine 20 mcg/kg/min. Dobutamine produced identical changes in cardiac output, stroke volume, and systemic vascular resistance even during severe lactic acid acidosis (pH 7.0, PCO2 40 mm Hg). Dopamine, however, failed to increase cardiac output and stroke volume and to decrease systemic vascular resistance during lactic acidosis. The plasma norepinephrine level was elevated from 0.49 to 3.01 ng/ml during normal acid-based state and from 1.76 to 9.53 ng/ml during severe lactic acid acidosis by the infusion of dopamine. Dobutamine infusion did not affect the plasma norepinephrine level during normal acid-base state but reduced the level during lactic acid acidosis. The marked increase in plasma norepinephrine following dopamine infusion may explain both the decrease in cardiac output and the increase in systemic vascular resistance in response to dopamine infusion during severe lactic acid acidosis. These results indicate that dobutamine may be more useful than dopamine in improving cardiac output during severe acidosis.