The effects of intravenous nitroglycerin (TNG) were evaluated in 39 patients with a first acute myocardial infarction, subdivided according to Killip into those with left ventricular failure (N = 24) and those without (N = 15). A group of 38 randomly selected patients treated in a conventional, but unstandardized manner, served as a control (C). TNG caused a statistically significant reduction in pulmonary artery diastolic pressure, regardless of its initial value, by about 30%. Neither cardiac index nor total peripheral resistance was significantly changed. Infarct size, measured in gEq of isoenzyme MB creatine kinase (CK-MB), was smaller by about 40% in both subgroups of patients treated with TNG, when compared with the controls. A significant difference was found in peak CK-MB blood levels only in the group of patients with left ventricular failure (Killip classes II and III) treated with TNG. The best results were obtained when TNG was given not later than 4 h after the onset of the symptoms of infarction.