Several studies have shown that infarct size is reduced following thrombolytic treatment in patients with acute myocardial infarction. Exercise test variables, such as an impaired heart rate response during exercise, are known to be related to left ventricular function and patient prognosis following acute myocardial infarction. The present study was performed to compare exercise test variables in acute myocardial infarction patients following either intravenous thrombolysis or placebo. Symptom-limited bicycle ergometer tests, carried out 1-2 weeks from the infarction, were performed in 85 patients randomized to intravenous streptokinase (N = 41) or placebo (N = 44) given within 12 h from onset of symptoms. At rest heart rate, systolic blood pressure and rate-pressure product were similar in the two groups. At maximum workload the streptokinase treated patients had a higher median maximal heart rate than controls (136 vs. 126 b.min-1, P less than 0.01) but only a trend towards higher systolic blood pressure was seen (175 vs. 163 mmHg, P = 0.09). Rate-pressure product at maximal exercise was 23,620 vs. 20,100 mmHg.b.min-1 respectively, (P less than 0.01). Total exercise time, ST-segment deviation, occurrence of angina pectoris and left ventricular ejection fraction were similar in the two groups. The trend towards an increased heart rate at maximum workload in streptokinase-treated patients was seen at all levels of left ventricular ejection fraction, and at all levels of exercise capacity.(ABSTRACT TRUNCATED AT 250 WORDS)