The effect of i.v. nitroglycerin administration on indexes of infarct size was examined in 31 patients with acute myocardial infarction. Serial creatine kinase (CK) and CK-MB isoenzyme determinations were used to calculate infarct size. Twenty-nine patients served as controls. Two subgroups of the study group were formed to evaluate differences between early and late intervention. In the first group (n = 22), continuous infusion of nitroglycerin over 48 hours was initiated within 8 hours (mean 4.5 hours) after the onset of symptoms. Peak CK activity for the nitroglycerin-treated patients (n = 9) in this subgroup was 544 U/1 vs 871 U/1 for the controls (n = 13) (p less than 0.05). The rate of CK release was reduced from 79 to 33 U/1.hr (58%), as was total CK and CK-MB release (p less than 0.02). Calculated infarct size was 69 gEq in the controls and 48 gEq in patients receiving nitroglycerin (CK-MB: 69 vs 43 gEq, p less than 0.05). In the late intervention subgroup, nitroglycerin therapy was begun more than 8 hours (mean 12.8 hours) after the onset of symptoms. Here, too, use of the agent was associated with lower peak CK and CK-MB levels as well as a reduction in calculated infarct size (p less than 0.05). Hemodynamic measurements, recorded every 4 hours, showed that nitroglycerin also reduced left ventricular filling pressure significantly and cardiac output increased. Blood pressure fell slightly, and systemic vascular resistance declined. The results indicate that i.v. nitroglycerin reduces CK and CK-MB release and thus calculated infarct size in both early and late intervention.