Among the different endothelin (ET) isoforms, ET-3 has been reported to exhibit a less potent constrictor activity than ET-1 and ET-2. Furthermore, distinct endothelin receptor subtypes have been identified in several tissues or cell types. In this study, we investigated the binding characteristics of the three endothelin isoforms in cultured rat aortic smooth muscle cells. [125I]ET-3 exhibited an apparent affinity and a number of binding sites 10 and 6 times smaller, respectively, than [125I]ET-1 and [125I]ET-2. In contrast to ET-1 and ET-2, ET-3 appeared to elicit a reversible binding and did not modify ET-1 binding characteristics in receptor-regulation experiments. In competition experiments ET-1 and ET-2 equally inhibited the binding of the three endothelin isoforms, whereas ET-3 was less potent in competing with [125I]ET-1 and [125I]ET-2 than [125I]ET-3. These results suggest that rat aortic smooth muscle cells possess 2 subtypes of endothelin receptors (A and B) differing by their affinity for ET-3, their proportion, the reversibility of the binding and their sensitivity to down-regulation.