The antihypertensive effect and the tolerability of the cardioselective beta-blocking drug metoprolol, in comparison to methyldopa, were assessed in 119 hypertensive patients (73 WHO stage 1 and 46 WHO stage 2). After 2 weeks of placebo wash-out, the patients were randomly allocated to treatment with either of the two drugs: metoprolol up to 200 mg bid, and methyldopa up to 500 mg bid, for 6 weeks. Periodical clinical, biochemical, haematological, radiological and electrocardiographical measurements were performed. In respect to pre-treatment values, heart rate, both in lying and standing position, was significantly reduced (P less than 0.01) only in the metoprolol group, while systolic and diastolic blood pressures were significantly reduced (P less than 0.01) with both drugs in both positions. Asymptotic regression analysis showed that velocity of blood pressure reduction was comparable with both drugs. In the lying position, the diastolic blood pressure reduction obtained with metoprolol was significantly greater (P less than 0.05) in respect to that obtained with methyldopa. In general, side effects were few and of mild severity: mainly bradycardia in the metoprolol group and dizziness and fatigue in the methyldopa group. After the formal end of the double-blind trial, 36 patients, 14 in the metoprolol group and 22 in the methyldopa group, were treated in open conditions with metoprolol alone; after an average period of 6.5 weeks, diastolic blood pressure was significantly reduced (P less than 0.05) only in the group previously treated with methyldopa. In conclusion, metoprolol is a well tolerated and effective antihypertensive agent, which may be safely used in patients with mild to moderate hypertension.