The Hypertension and Clinical Hemodynamics Section, Veterans Administration Hospital, and the Department of Medicine, Georgetown University School of Medicine An isoquinoline derivative, esproquin (NC 7197), was administered in single oral doses of 100, 200, or 300 mg to 15 normal volunteers. A dose-related positive inotropic effect was characterized by an increase in cardiac output and stroke volume that persisted for about 4 hours without a significant change in heart rate associated with a reduction in pre-ejection period and an increase in mean systolic ejection rate. Arterial pressure rose with the lower doses, but diastolic pressure fell and forearm blood flow increased with the highest dose. These data indicate that oral administration of esproquin produces a sustained positive inotropic effect and that in larger doses peripheral vasodilatation results either from adrenergic blockade or a direct vascular effect. The drug deserves evaluation for the treatment of chronic heart failure.