An isoquinoline derivative (NC 7197) was administered intravenously in a dose of 0.2 to 1.0 mg. per kilogram of body weight in four normal subjects and in 18 patients with shock or heart failure. An increase in cardiac output averaging 45 per cent in the normal subjects and 47 per cent in the patients was sustained for at least two hours. Heart rate was only slightly increased and stroke volume consistently rose. Arterial pressure was unchanged in normals but rose in the patients, although peripheral vascular resistance usually fell. Forearm blood flow rose only with the higher doses but renal and hepatic flow rose in the two patients studied. Norepinephrine-induced forearm vasoconstriction was not blocked during intra-arterial administration of NC 7197. Infusion into the left circumflex coronary artery of dogs led to a localized increase in myocardial contractility which was blocked by propranolol or pretreatment with reserpine. These data indicate that NC 7197 is a potent inotropic drug of prolonged duration of action which acts through an adrenergic mechanism and may be useful in the treatment of low output states.