The hemodynamic effects of a single oral dose of 20 mg of nifedipine were studied in 21 stable patients after an acute myocardial infarction before withdrawal of Swan-Ganz and intra-arterial catheters. The drug appears safe, and no significant untoward effects were noted. Significant hemodynamic changes were present between 15 and 120 minutes after ingestion with a peak at 60 minutes. Mean arterial pressure decreased from 81+/-2.7 (SEM) to 71+/-2.3 mm Hg (P<0.001), and systemic vascular resistances decreased from 1438+/-88 to 1144+/-63 dynes/sec/cm5 (P<0.001). Cardiac index increased from 2.7+/-0.1 to 3.1+/-0.1 L/min/sq m (P<0.01) and heart rate from 79 +/- 3 to 82 +/- 4 (P<0.01). The pressure-rate product decreased from 10.3 X 10(3) to 9.5 X 10(3) (P<0.05), and pulmonary wedge pressure was unchanged from 12 mm Hg. The hemodynamic changes were similar whether patients were receiving propranolol or not. The afterload reducing effect was potentially most beneficial in the subgroup of patients with depressed left ventricular function where cardiac index increased by 20 percent.