Concentrations of serotonin and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in platelet rich plasma, the urinary 5-HIAA excretion rate, and serotonin-induced platelet aggregation were measured in 17 patients with essential hypertension before, and at the end of, 8 weeks of oral ketanserin therapy at 20 to 40 mg twice daily. Ketanserin lowered systolic and diastolic blood pressure (p less than 0.01) and led to a reduction of serotonin concentration in platelet rich plasma in all patients (p = 0.05), as well as a decrease in 5-HIAA excretion rates in patients older than 55 years (p less than 0.05). Changes in 5-HIAA concentration in platelet rich plasma correlated with the fall in diastolic blood pressure (r = 0.67, p less than 0.05). Serotonin-induced platelet aggregation was inhibited by ketanserin (p less than 0.05), and this was more pronounced in older patients. Thus, antihypertensive therapy with ketanserin reduced platelet aggregation and serotonin metabolism in relation to the age of patients, and this may contribute to the reduction of their elevated rates of thromboembolic complications.