In order to study the circulatory effects of practolol in the depressed heart, myocardial infarction was produced in eight dogs by ligation of multiple coronary artery branches. Significant decreases in cardiac output, stroke volume, and left ventricular dp/dt, concomitant increases in systemic vascular resistance and left ventricular end-diastolic pressure occurred following ligation. The thoracic cavity was closed and the animals were allowed to stabilize their circulatory parameters. Practolol was then given in intravenous doses of 250, 500 and 1,000 µg/kg. These doses resulted in blood levels of 0.18, 0.30 and 0.62 µg/ml, without producing further deterioration of any of the hemodynamic variables that had been depressed by multiple coronary ligation. There was, however, a significant decrease in heart rate without a drop in cardiac output, which resulted in a slight but significant rise in stroke volume following practolol administration. Thus, it is concluded that practolol did not further reduce an already compromised heart in this model of acute infarction.