Prazosin is believed to have an antihypertensive action due to a selective blockade of alpha-adrenoceptors on vascular smooth muscle. Using an in vitro autoradiographic technique specific binding of [3H]prazosin to the abdominal aorta and renal artery in the rat has been shown. The binding of [3H]prazosin to arterial vessels may provide some evidence for the site where prazosin exerts its antihypertensive action.