The effect of quinapril and nifedipine on platelet aggregation, vascular endothelial function and coagulation system activity, was compared in a parallel-group, investigator-blind study carried out on patients with mild to moderate hypertension but no other diseases or receiving medication which might affect platelet function, vascular endothelium or coagulation. Forty patients (two groups of 20 patients each) and 20 control subjects were recruited. Patients were randomised to receive either quinapril or nifedipine retard and the dose escalated to control hypertension. Platelet aggregation studies were assessed serially and beta-thromboglobulin, angiotensin-converting enzyme (ACE), von Willebrand factor (vWF) coagulation factors VIIIc, XII and fibrinogen were measured at the beginning and end of the 12-week period. Blood pressure was adequately controlled in all patients in both groups. Platelet function was impaired in certain parameters (slope of the reaction with ADP and collagen and maximum aggregation with collagen) in the patient group compared to controls before treatment and this improved in patients on quinapril but not on nifedipine; likewise beta-thromboglobulin was higher in the patient group and fell significantly in the quinapril group but not those on nifedipine. Measurements of endothelial function and coagulation were normal before treatment and showed no alteration during the study, except in the expected fall in plasma ACE in the quinapril group. The results indicate that the ACE inhibitor, quinapril, has a beneficial effect on platelet function unlike the calcium channel blocker, nifedipine.