The authors compared the relative safety and efficacy of changing treatment from once-daily atenolol to metoprolol in patients with essential hypertension. A parallel-group randomized clinical trial was conducted in two phases: a 4-week baseline single-blind phase using atenolol 50 mg, followed by a 4-week randomized double-blind treatment phase using either atenolol 50 mg or metoprolol 100 mg administered once daily at noontime. Patients with well-controlled hypertension already prescribed 50 mg of atenolol (with or without the addition of a diuretic) for control of hypertension were selected for participation from the outpatient hypertension clinic of the Department of Veterans Affairs Medical Center, Pittsburgh, Pennsylvania. Seated blood pressure (BP) and pulse were obtained during the baseline phase and during the randomized treatment phase. Twenty-four-hour ambulatory BP monitoring was performed once during the baseline phase and once during the randomized treatment phase, near the end of each 4-week period. There were no within- and between-treatment differences in office systolic and diastolic BP. There was a slight increase in pulse (average = 5.2 beats/minute; P = .02) for those participants treated with metoprolol. For within-treatment groups, the ambulatory BP data showed no significant differences in systolic and diastolic BPs, except for an increase in morning diastolic BP for those randomized to metoprolol (average = 6.2 mm Hg; P = .01). For between-treatment groups, the metoprolol arm had a higher morning systolic BP (P = .01), a higher morning diastolic BP (P = .03), and a higher nighttime heart rate (P = .01).(ABSTRACT TRUNCATED AT 250 WORDS)