To evaluate the ex-vivo effects of labetalol, at effective vasodepressor doses, on platelet aggregation in hypertension, 30 hypertensive patients were randomized to either a labetalol or control group in a ratio of two to one. For the labetalol group, the dosage was titrated from 400 mg to 600 mg per day to achieve a fall of 10 mmHg in mean arterial pressure (MAP). No drug was used in the control group. The study lasted for 4 to 8 weeks. At the beginning and end of the study, platelet aggregation to collagen, adenosine diphosphate and epinephrine were measured according to Born's method. In contrast to the significant reduction in MAP, platelet aggregation was not altered by labetalol compared with the control group. Our data do not support the hypothesis that labetalol, at effective vasodepressor doses, inhibits platelet aggregation in mild-to-moderate hypertensive patients.