The authors tested in an open, uncontrolled trial in a group of 23 patients with essential hypertension grade I-II (WHO classification) the effect of Metoprolol OROS. The OROS system is a new form of Metoprolol administration which makes it possible to maintain by a single dose per day a steady plasma concentration, while preserving the cardioselectivity and total 24-hour effectiveness during treatment of hypertension and angina pectoris. After eight weeks of Metoprolol OROS administration, in doses gradually adjusted to the therapeutic action, gradually a significant decrease of the heart rate (HR) occurred, of the systolic blood pressure (BPs) and diastolic blood pressure (BPd) (p less than 0.01 for all values) in a recumbent as well as upright position. A reduction of the BPd in an upright position by greater than or equal to 10 mm Hg was achieved in 85% of the patients, in 73.9% of the patients the BPd in an upright position dropped below 95 mm Hg. Four patients developed side-effects which were mild to medium severe (vertigo, palpitations, fatigue, sensation of tremor, tension in the lower extremities). Two patients discontinued treatment early, the main reason in both being palpitations which were under better conversely, in two patients palpitations which were not adequately controlled by previous metoprolol treatment, disappeared completely during Metoprolol OROS treatment. During the trial no significant changes in the investigated laboratory values incl. total cholesterol were recorded, Metoprolol OROS administered once per day is an effective, safe and well tolerated preparation in treatment of mild to medium severe essential hypertension.