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The effects of intravenous metoprolol and propranolol on left-ventricular performance were studied by radionuclide cardiography in eight healthy young men at rest and during exercise. At rest, a fall of 7 to 10 mmHg was recorded in systolic blood pressure after both drugs (p<0.05). The decrease in cardiac output entirely ascribed to relative bradycardia was 18% after propranolol (p<0.01) compared with 6% after metoprolol (p<0.05). Left-ventricular volumes did not change at rest. Systemic vascular resistance increased by 14% after metoprolol and by 28% after propranolol (both p<0.05). During submaximal bicycle exercise the systolic blood pressure fell by 15 mmHg after both drugs (p<0.05). The reduction in exercise heart rate was 17 beats min'1and 18 beats min−−1(both p<0.01), but propranolol reduced cardiac output by 18% (p<0.01) compared with 5% (p<0.05) after metoprolol, reflecting a reduction in stroke volume induced only by non-selective blockade. A more pronounced left-ventricular end-diastolic dilatation during exercise was allowed by metoprolol and the fall in the exercise ejection fraction was, therefore, smaller after this drug. Concomitantly, the calculated systemic vascular resistance was increased only 6% by metoprolol (p<0.05) compared with a 29% increase by propranolol (p<0.01). These findings suggest that, at least in healthy subjects, metoprolol is superior to propranolol in the ability to limit adverse cardiovascular changes.