The effects of dihydropyridine calcium antagonists on blood pressure and platelets, and the effects of aspirin on the circadian antihypertensive efficacy of dihydropyridines and on the 24-h platelet-activity profile, were the focus of a double-blind study. Patients with essential hypertension were treated for 8 weeks with either isradipine focus of double-blind study. Patients with essential hypertension were treated for 8 weeks with either isradipine (2.5 mg/day) or nitrendipine (10 mg/day). Aspirin (100 mg/day) was added to both treatment groups for a further 8 weeks. Measurements were taken after 4 weeks of placebo, after 8 weeks of dihydropyridine treatment, and after 8 weeks of treatment combined with aspirin. Plasma levels of beta-thromboglobulin (beta-TG) and platelet aggregation induced by serotonin were measured six times during 24 h. Both dihydropyridines significantly lowered systolic and diastolic blood pressure. The addition of aspirin to dihydropyridine treatment had no significant effects on systolic or diastolic blood pressure. Dihydropyridine treatment lowered the increased 24-h plasma beta-TG profile and inhibited platelet aggregability. Aspirin added to nitrendipine led to a further significant decrease in beta-TG levels whereas its addition to isradipine was accompanied by a partial increase in plasma beta-TG. It is concluded that increases in platelet activity in hypertensive patients can be prevented with either isradipine alone or nitrendipine plus aspirin. Aspirin in a daily dose of 100 mg does not affect the antihypertensive efficacy of calcium antagonists.