To determine the effect of metoprolol on silent ischemia and platelet aggregability, 10 patients with coronary artery disease were studied with a randomized, double-blind, placebo-controlled, crossover trial. Patients were treated with metoprolol (200 mg b.i.d.) or placebo for 1 week and then received the alternate therapy. Two days before the end of each treatment period, platelet aggregability was studied for 24 hours, and a 48-hour ambulatory electrocardiogram was obtained. Compared with placebo, metoprolol significantly decreased the total number (from 26 to 4, p less than 0.1) and duration (from 735 to 84 minutes, p less than 0.01) of silent ischemic episodes. This decrease was accompanied by a decrease in the mean blood pressure (from 127/81 to 118/71 mm Hg, p less than 0.01) and the mean heart rate (from 70 to 54 beats/min, p less than 0.01). The incidence of silent ischemic episodes in the morning was significantly higher in untreated patients than in treated patients. The few episodes observed during metoprolol treatment occurred at the same time as the peak incidence observed during placebo treatment. During placebo treatment, platelet aggregability increased from 6:00 to 9:00 AM as reflected by a decrease in the threshold concentrations of ADP and epinephrine required to induce biphasic platelet aggregation (from 4.8 +/- 0.8 to 2.6 +/- 0.4 microM, p less than 0.02; and from 7.3 +/- 2.3 to 1.8 +/- 0.9 microM, respectively, p less than 0.02). Metoprolol did not alter the basal level nor blunt the morning increase of platelet aggregability.(ABSTRACT TRUNCATED AT 250 WORDS)