Independent trials of early administration of beta-blockers and thrombolytic agents have shown beneficial effects on both short- and long-term prognoses in acute myocardial infarction (AMI). The effects of a combination of the two strategies have not been thoroughly documented. Three hundred and fifty-two patients, of less than 75 years of age, with chest pain indicative of AMI, and onset less than 2 h and 45 min before first examination, were randomized to treatment with rt-PA or placebo. All patients without contraindication were given intravenous metoprolol 15 mg acutely and then 200 mg orally daily. Treatment was started either at the prehospital stage or in hospital. Thirty-seven per cent of patients had contraindications to beta-blockade, the most frequent of which were heart rate less than 60 beats min-1 and hypotension. The remaining 63% were given intravenous beta-blockade. No side-effects of metoprolol, alone or in combination with rt-PA, were observed during the prehospital phase. Overall, toleration of the treatment was good. Reduction in enzymatically estimated infarct size by rt-PA was more pronounced in patients who were also treated with metoprolol (41%, P less than 0.001) than in those with contraindications to beta-blockade (15%, NS). Patients who were also treated with metoprolol also had a lower incidence of Q-wave infarctions, congestive heart failure and ventricular fibrillation than those who were not given intravenous beta-blockade. In conclusion, toleration of intravenous administration of rt-PA and metoprolol was good, and this was also the case in the prehospital phase.(ABSTRACT TRUNCATED AT 250 WORDS)