A single-blind, placebo-controlled study was conducted to compare the inhibitory effects of intravenous propranolol on exercise-induced tachycardia in eight matched pairs of healthy black and white volunteers, consisting of equal numbers of men and women. At the beginning of each test session, subjects were exercised on a bicycle ergometer--the load predetermined to increase their heart rates to 160-190 beats/min. Exactly 30 min after having received an intravenous injection of placebo or propranolol (the dose varying from 0.0125 to 0.15 mg/kg), they were subjected to the same exercise load. Heart rates were continuously recorded throughout exercise periods, and readings at 6, 8, and 10 min were compared for pre- and postinjection periods. A washout period of at least 3 days was allowed to elapse between sessions. Postinjection reductions of heart rate in blacks were compared with postinjection reductions in their white counterparts. Analysis of results indicated that the dose-response curve for the heart rate reduction resulting from propranolol for blacks was shifted to the right with respect to that for whites. In clinical terms, this means that to achieve the same degree of beta-blockade in blacks, a larger dose of propranolol is required than for whites, but that the same maximum is attainable.