Excised segments of rabbit middle cerebral artery were cannulated and perfused in vitro at constant flow (1 ml/min). In vessels with intact endothelium, integrity of the intima was checked by determining relaxations to acetylcholine. In another series of segments, the endothelium was destroyed mechanically. Serotonin (5-HT) was added to the perfusate in increasing concentrations (3 x 10(-10) to 10(-4) M). Perfusion pressure served as the index of vasomotor responses of the arterial segment. At therapeutic concentrations, the 5-HT2 antagonist naftidrofuryl did not change perfusion pressure of normal vessels in the absence of 5-HT. Addition of 5-HT to intact vessels had no effect on perfusion pressure, but potentiated the contraction induced by KCl. This reaction was greatly amplified after de-endothelialization. Addition of naftidrofuryl (10(-7) M) to the perfusate strongly inhibited 5-HT's contracting effects on both intact and deendothelialized arterial segments. Naftidrofuryl at 10(-6) M further displaced the dose-effect curve to the right in a competitive manner. These data confirm the potentiating effect of 5-HT on vascular smooth muscle (even in intact vessels) and the anticonstricting effects of naftidrofuryl when 5-HT2 receptors on smooth muscle cells are activated by serotonin.