The effects of calcium antagonists on platelets, and the effects of aspirin on the antihypertensive efficacy of calcium antagonists and on the 24-h platelet-activity profile were investigated in a double-blind study. Patients with essential hypertension were treated for 8 weeks with either nitrendipine (10 mg once daily) or isradipine (2.5 mg once daily). Aspirin (100 mg once daily) was added to both treatments for a further 8 weeks. Measurements were taken after placebo, after 8 weeks of active treatment, and after 8 weeks of treatment plus aspirin. Plasma levels of beta-thromboglobulin (beta-TG) and platelet aggregation (PA) were measured six times over 24 h. Isradipine and nitrendipine significantly lowered systolic and diastolic blood pressure. The addition of aspirin had no effect on blood pressure. Platelet activity before treatment exhibited circadian variations with the lowest values of beta-TG and PA at 0530 h and the steepest increases between 0530 and 0900 h. Although both calcium antagonists decreased beta-TG levels (P < .05), the effect with isradipine was more pronounced than that with nitrendipine (P < .05). Aspirin added to nitrendipine produced a significant decrease in beta-TG levels whereas isradipine plus aspirin was accompanied by a partial increase in beta-TG. It is concluded that hypertensive patients exhibit circadian increases in platelet activity that can be prevented by either isradipine alone or by treatment with nitrendipine plus aspirin. Chronic aspirin intake at a daily dose of 100 mg does not affect calcium antagonist antihypertensive efficacy.