The aim of this study was to determine whether oxidative stress occurs in patients that received thromboyltic therapy. In 16 patients with acute myocardial infarction, erythrocyte superoxide dismutase (SOD) and glutathione peroxidase (GSHPx) activities (GSH) levels were measured before and 5 min, 1 h and 24 h after streptokinase infusion. The same parameters were studied upon admission, after the chest pain declined, 1 h and 24 h later in seven patients with acute myocardial infarction. In the treatment group, SOD activity and GSH level were significantly decreased during the first 24 h after an attack (mean SOD activities were 2,889, 2,566, 2,606, 2,485 U/g Hb, the mean GSH levels were 64.36, 56.41, 54.28, 55.88 mg dL(-1) erythrocyte, respectively). GSHPx activities were not changed after the thrombolytic therapy. SOD, GSHPx and GSH levels were not significantly altered in untreated patients during the observation period. Our findings suggest that in acute myocardial infarction, thrombolytic therapy significantly decreased erythrocyte SOD activity and GSH level, possibly due to the generation of reactive oxygen species.