The hemodynamic effects of petrinitrol, a new antianginal agent, were evaluated in 30 patients during cardiac catheterization. The administration of 5 mg and 10 mg doses, both sublingually and orally, caused significant reductions of systemic arterial pressure, cardiac index, stroke index, and pulmonary arterial pressure that were prominent 15 to 30 minutes after administration of the drug. All but the 5 mg oral dose produced an increment of heart rate. The 10 mg sublingual dose also caused a significant reduction of left ventricular work. Total systemic resistance, and the product of blood pressure and heart rate did not change significantly. It appears that petrinitrol and nitroglycerin have several similar effects upon the cardiovascular system; but in the doses administered, petrinitrol caused changes of greater magnitude and longer duration.