In a randomized double-blind multicenter study in 100 hypertensive patients, the effect of once daily (od) dosing with a new controlled release (CR/ZOK) formulation of metoprolol was compared with that of twice daily (bid) dosing with metoprolol conventional tablets. Eligible patients had a resting seated diastolic blood pressure (DBP) greater than or equal to 95 mm Hg and less than 110 mm Hg at the end of a 6-week open placebo run-in. The active treatment phase lasted 8 weeks. The starting dose was 100 mg od in the CR/ZOK group (N = 53) and 50 mg bid in the tablet group (N = 47). The dose was increased to 200 mg od and 100 mg bid, respectively, in nonresponders (DBP greater than 95 mm Hg) at the end of the first 4-week period. Approximately 40% of both groups received concomitant diuretic therapy throughout the study. The SBP, DBP and HR were reduced compared to baseline in both treatment groups after 4 and 8 weeks. After 4 weeks, 85% of the CR/ZOK group and 74% of the tablet group had DBP less than 95 mm Hg. After another 4 weeks, the corresponding figures were 93% and 93%. There was no statistically significant difference between the treatment groups in the decrease in either SBP, DBP or HR, nor was there any difference in the percentage of responders. Both treatments were equally well tolerated. In conclusion, the antihypertensive effect of once daily dosing (100-200 mg) with the new CR/ZOK formulation of metoprolol is as effective as that of twice daily dosing (50-100 mg) with conventional metoprolol tablets.