In two separate but similarly designed placebo-controlled studies, the efficacy of metoprolol CR/ZOK 50 mg once daily was evaluated in 64 patients (mean age 53 years) with mild hypertension (Study I) and in 34 elderly patients (mean age 68 years) with mild to moderate hypertension (Study II). Both studies were of randomized, double-blind, parallel-group design, the entry criteria for diastolic blood pressure being greater than or equal to 90 less than 105 mm Hg in Study I and greater than or equal to 95 less than 110 mm Hg in Study II. Study I: The impact of metoprolol CR/ZOK 50 mg once daily for 4 weeks compared with placebo 24 hours post dosing was to produce a significant reduction in supine (P = .0001), and standing (P less than .0001) systolic blood pressures and the standing diastolic pressure (P = .035). The supine diastolic pressure was lower after metoprolol CR/ZOK but not significantly so. Study II: Metoprolol CR/ZOK 50 mg daily given to elderly hypertensives when compared with placebo 24 hours post dosing produced a significant fall in supine (P = .022) and standing (P = .022) diastolic pressure. Systolic pressure and heart rate were not significantly reduced. One patient in Study II had a nonfatal myocardial infarct whilst receiving placebo. There were no other serious adverse effects in either study and no patients were withdrawn from the trial because of drug related unwanted events. The studies indicate that 50 mg metoprolol CR/ZOK may be an effective antihypertensive agent which may prove useful in the treatment of mild to moderate hypertension, particularly in the elderly.