The antihypertensive effects of once-daily administration of celiprolol 400 mg were compared with those of once-daily enalapril 20 mg in 20 mild to moderate essential hypertensives in a single-blind study in which the subjects were randomized to 2 weeks' treatment with either drug, preceded by 2 weeks of placebo administration. Supine and standing systolic blood pressure (SBP), diastolic pressure (DBP), and heart rate (HR) were measured at rest and during exercise (exercise bicycle with increasing work loads up to 100 watts) at the end of either period, 20 to 24 hr after drug or placebo administration. Compared to placebo, both celiprolol and enalapril reduced resting supine and standing SBP and DBP to a marked and similar extent; resting HR was unaffected by enalapril, while celiprolol produced modest reductions. During exercise, however, celiprolol significantly attenuated rises in SBP and HR, while enalapril did not. Indexes of cardiac function at rest obtained by echocardiography and systolic time intervals were unaffected by either drug. Thus, despite similar reductions by both drugs in resting blood pressures, celiprolol produced better antihypertensive effects during exercise than enalapril.