The mechanism of blood pressure reduction by ketanserin in hypertensive patients is still not fully understood. It can occur in the absence of evident alpha 1-adrenergic blockade. However, particularly with prolonged therapy, features of alpha 1 blockade appear. There are both similarities and differences between the pattern of effects caused by ketanserin and prazosin. A central mode of action of ketanserin contributing to the blood pressure reduction is not excluded. It appears unlikely, however, that inhibition of aldosterone secretion makes a substantial contribution to the antihypertensive effect of ketanserin in humans.