The hemodynamic and hormonal responses to nitroglycerin administered transdermally in a gel-like matrix were evaluated in nine patients with severe congestive heart failure and in nine normal subjects. In normal subjects, peripheral vasodilation was accompanied by reflex sympathetic stimulation as reflected by an increase in heart rate and plasma norepinephrine. In patients with heart failure, nitroglycerin produced sustained hemodynamic effects that began 30 minutes after the application and fully persisted for at least 6 hours. A significant decrease in right and left ventricular filling pressures was associated with an increase in stroke index and a significant decrease in forearm and pulmonary vascular resistances. There was no change in heart rate and systemic arterial pressure or in plasma norepinephrine or plasma renin activity. After 24 hours, pressures had partially returned to control levels, but mean pulmonary artery pressure was still significantly lower than in the control period. After removal of the nitroglycerin, each patient exhibited a decrease in cardiac index and an increase, above the control values, in pulmonary and systemic arterial pressures and pulmonary, systemic and forearm vascular resistances. This transient rebound appeared to be unrelated to stimulation of the sympathetic or renin-angiotensin systems. Thus, transdermal absorption of this new form of nitroglycerin appears to provide a nitrate vascular effect that is sustained for 24 hours, but an endogenous vasoconstrictor effect may influence the hemodynamic response over the first 24 hours.