In a double-blind study with parallel groups 195 hypertensive patients were randomly allocated to treatment with either conventional tablets of metoprolol, 100 mg once daily, or a new controlled-release (CR) formulation of metoprolol, 100 mg once daily. The dose was doubled if the patient's diastolic blood pressure remained greater than or equal to 95 mmHg after six weeks on 100 mg, whereas well-controlled patients continued on 100 mg once daily for a further six-week period. In the metoprolol tablet group the 200 mg dose was administered in the form of Durules. There was a significant reduction from the placebo baseline in systolic and diastolic blood pressure and heart rate at 24 h after both six weeks and 12 weeks of active treatment; no significant difference in the mean reduction from baseline between the two groups was demonstrated. However, significantly more patients responded to treatment with metoprolol CR when compared with those patients taking metoprolol tablets. After six weeks of active treatment 61% of the metoprolol CR group and 56% of the conventional metoprolol tablet group had a diastolic blood pressure less than 95 mmHg. After another six weeks the corresponding figures were 83% and 69% respectively. Between week 6 and 12, 36% of patients in the metoprolol CR group and 42% of patients in the conventional metoprolol tablet group were receiving a 200 mg dose. All formulations of metoprolol were well-tolerated. Fewer subjective symptoms were reported during active treatment than during the placebo phase. There were no differences between the groups with regard to changes in laboratory variables from baseline, changes in all combined symptoms, or changes in any one symptom.