The antihypertensive effect of daily doses of three beta-adrenoblockers (Bisoprolol, 10 mg once a day, propranolol, 80 mg twice a day, and methoprolol, 100 mg twice a day), and placebo was examined in 14 patients with persistent mild and moderate hypertension during a double blind cross-over study by using 24-hour monitoring of blood pressure and its routine measurements. The latter made by a mercury sphygmomanometer indicated that the antihypertensive and negative chronotropic effect of Bisoprolol in a dose of 10 mg remained 24 hours after its administration and it did not significantly differ from that of the two other agents given in the above doses. The application of 24-hour blood pressure monitoring allows a more pronounced antihypertensive effect of bisoprolol to be revealed during 24 hours than that displayed by the two agents. Bisoprolol is an effective and safe antihypertensive agent.