Exercise test variables, such as an impaired heart rate response, 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 infarct patients following either intravenous thrombolysis or placebo. Symptom-limited bicycle ergometer tests, carried out one to two weeks from the infarction, were performed in 85 patients randomized to intravenous streptokinase (n = 41) or placebo (n = 44) given within 12 hours from onset of symptoms. Resting heart rate, systolic blood pressure and rate-pressure product were similar in the two groups. At maximum workload the streptokinase treated patients had a significantly higher median maximal heart rate than controls (136 vs. 126 min-1; p <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 x min-1; p <0.01). A significantly smaller number of patients in the streptokinase group had exercise capacity below 50 W (0% vs. 15.9%; p <0.01). In conclusion: patients treated with intravenous streptokinase for acute myocardial infarction reach both higher heart rates and rate-pressure products at maximum workload than their controls thus indicating that the beneficial effects of thrombolysis after acute myocardial infarction are reflected in an improved heart rate response during exercise.