In 38 patients with acute myocardial infarction the effect of nitroglycerin on infarct size was studied. Patients were randomized into two groups. 16 patients received continuous nitroglycerin infusions of 0.6 to 6.0 mg/h (mean 2.3 mg/h) over a 48 h period, 22 patients received no specific therapy and served as control. Nitroglycerin was given in the mean 12 +/- 5 (+/- 1 SD) hours following onset of chest pain and 8 +/- 5 after the increase of CK values. Infarct size was determined according to the time activity curve of creatine kinase (CK) and of its myocardial isoenzyme (CK-MB). In all but one patient hemodynamic parameters (left ventricular filling pressure, blood pressure, cardiac index) were measured. The mean infarct size was 51 +/- 30 CK-g-equiv. in control patients, and 48 +/- 33 g in nitroglycerin treated patients. Infarct size as calculated from CK-MB values was 60 +/- 36 g (n=16) in control, and 52 +/- 41 g (n=11) in treated patients. At left ventricular filling pressure values (LVFP) below 20mm Hg infarct size amounted to 43 +/- 30 g (n=12) in control, and to 41 +/- 32 g (n=11) in the nitroglycerin group. At LVFP values above 20 mmHg infarct size was 61 +/- 29 g (n=10) in control as opposed to 64 +/- 32 g (n=5) in treated patients. There was no difference between infarct size as predicted during the first 7 h and the observed infarct size. - Despite the known beneficial effect of nitroglycerin on hemodynamics and on myocardial ischemia, infarct size seems not to be greatly reduced, however, intervention occurred fairly late (12 h). In early intervention beneficfial effects seem likely.