N-acetylprocainamide (NAPA) is the active metabolite of procainamide currently undergoing evaluation for its antiarrhythmic properties. Its effects on hemodynamic variables and on ventricular function in man are poorly defined. The effects of intravenously administered NAPA (18 mg/kg over 30 minutes; mean plasma level 40.2 +/- 6.2 micrograms/ml) on hemodynamics and left ventricular ejection fraction (LVEF) assessed by radionuclide ventriculography were therefore determined in 14 patients undergoing diagnostic cardiac catheterization. The peak effects of the drug with respect to most measured variables were seen at 30 minutes and were still apparent at 60 minutes. NAPA increased heart rate (3% at 10 minutes; p less than 0.01), decreased mean pulmonary arterial pressure (14%; p less than 0.05) and capillary wedge pressure (27%; p less than 0.01), decreased mean arterial pressure (12%; p less than 0.01), cardiac index (8%; p less than 0.01), LV dp/dtmax (9%; p less than 0.05), and stroke work index (17%; p less than 0.01). The LVEF increased 5% (p less than 0.05). There was a trend for the systemic vascular resistance to decrease, but this did not reach statistical significance. The data show that NAPA exerts relatively weak peripheral arteriolar and venodilator effects associated with a mild reduction in contractility and cardiac output but an increase in LVEF in patients with preserved ventricular function.