The cardiovascular responses to intravenous doses of isoproterenol were measured in eight male volunteers before and during administration of 50 mg metoprolol, orally every 6 hours and 40 mg propranolol, orally every 6 hours for a total of five doses. The dose of isoproterenol required to produce an increase in heart rate of 25 beats/min (the ID25) was 2.0 +/- 1.4 microgram before beta blockade, 6.2 +/- 4.4 microgram during metoprolol, and 44.4 +/- 12.0 microgram during propranolol administration. Similar changes in diastolic blood pressure, QS2I, preejection period, and preejection period/left ventricular ejection time ratio occurred at the ID25 during treatment with both metoprolol and propranolol. In volunteers, propranolol produces a much more intense blockade of the inotropic and chronotropic effects of isoproterenol than does metoprolol.