The antihypertensive effect of the selective beta-1-adrenoceptor blocker, metoprolol, administered once daily was evaluated in 32 patients with primary hypertension. After a 4-wk placebo period, the patients were treated with either 150 mg or 300 mg of metoprolol, once daily, for 8 wk. Initially and during the treatment periods blood was drawn for analysis of metoprolol in plasma, plasma renin activity (PRA), and electrolytes, and urine was collected for determination of the urinary aldosterone excretion. Metoprolol reduced the blood pressure measured up to 26 hr after the last dose. The percentage of responders to metoprolol (decrease of mean arterial pressure greater than or equal to 10% over placebo) was 40% for patients on 150 mg and 71% for patients on 300 mg. Except in the standing position, heart rates were reduced for 26 hr after a 150-mg dose. There was a correlation between pretreatment PRA levels and antihypertensive effect of metoprolol in patients on 300 mg metoprolol but not in patients on 150 mg. Urinary aldosterone decreased equally during treatment in responders and nonresponders. Antihypertensive effects and side effects did not correlate with plasma metoprolol concentrations.