The antihypertensive effects of the new cardioselective beta-blocker celiprolol and acebutolol have been compared. Thirty patients with arterial hypertension WHO Grade I-II were treated in a double-blind fashion with celiprolol or acebutolol. Before starting the treatment and on Days 15 and 29, before the morning dose, blood samples were taken for measurement of the plasma level of celiprolol. At the same times physical examinations, and clinical and urine chemistry analyses were performed. At the 99% probability level both drugs had significantly lowered the systolic and diastolic blood pressures to normal values at the end of the second and fourth weeks. There was no significant difference between their antihypertensive efficacy. The decrease in diastolic blood pressure at the end of the second week was significantly correlated with the reciprocal of the plasma celiprolol concentration at steady-state at the end of the dosage interval.