Because it has been shown that high doses of propranolol (40 to 80 mg orally, four times daily) markedly attenuate cardiovascular response to exercise training in healthy subjects, the effects of lower doses of this nonselective beta-adrenergic receptor antagonist on the conditioning response were investigated. Twelve normal men underwent maximal treadmill testing before and after a 6 week intensive exercise program. After an initial test, six men were randomized in a paired fashion to receive low dose propranolol and the others received no drug. The average propranolol dose +/- standard error was 22 +/- 4 mg four times daily, and the average decrease in maximal heart rate due to propranolol was 32 +/- 4 beats/min. Both groups trained at comparable intensities. At the end of the training period, propranolol was stopped and testing was repeated so that the effect of beta-receptor blockade was no longer present but the training effects still persisted. Maximal oxygen consumption increased in control subjects from 47.5 +/- 1.1 to 51.4 +/- 0.4 ml/kg per min (p less than 0.05) but was unchanged in those receiving propranolol (47.2 +/- 1.9 versus 47.4 +/- 1.5). Exercise duration increased in both groups but the increment was greater in the control group (+2.4 versus +1.1 min, p less than 0.05). It is concluded that low level beta-receptor blockade attenuates cardiovascular conditioning in normal subjects in exercise training programs. High levels of sympathetic stimulation during training appear to be important, if not essential, to the conditioning process.