dl-nebivolol is a new, chemically unique, selective beta 1-adrenoceptor antagonist, without sympathomimetic activity. In the present study, the antihypertensive and negative chronotropic effects of 5 and 10 mg once-daily nebivolol were compared to those of 50 and 100 mg once-daily atenolol in 25 white, male subjects with essential hypertension, using a double-blind, crossover design, and a parallel placebo-treated group of subjects (N = 7). 24 hours after dosing, sitting and standing diastolic and systolic blood pressures and heart rates (at rest and during submaximal exercise) were reduced to the same extent by nebivolol and atenolol. On a weight-for-weight basis, nebivolol is ten times more potent than atenolol. Nebivolol and atenolol also reduced 1-hour ambulatory plasma renin activity. Once-daily nebivolol is an effective antihypertensive beta-adrenoceptor antagonist.