The central haemodynamic effects of metoprolol in patients with acute myocardial infarction and with heart rate less than or equal to 65 beats min-1 have been investigated in a randomized double-blind trial. The aim was to study the tolerance in this selected patient group and to assess possible differences in haemodynamic response amongst patients with initially higher heart rates. Exclusion criteria were: treatment with beta blockers; heart rate greater than 65 beats min-1; systolic blood pressure less than 110 mmHg; and physical signs of serious heart failure. Following pulmonary artery catheterization, 22 patients were randomized to metoprolol 15 mg i.v. + 50 mg q.i.d. orally (N = 12) or placebo (N = 10). Central pressures and cardiac output were recorded before and during the 24 hours after drug administration. There was a significant fall in heart rate, cardiac index, rate pressure product and stroke work index of 10-20% in the metoprolol, compared with the placebo group. The differences were most pronounced immediately after the metoprolol injection. The pulmonary artery capillary wedge pressure was not significantly changed. The overall haemodynamic response to metoprolol was similar to that reported in patients with acute myocardial infarction and heart rate above 65 beats min-1. Tolerance was good.