To clarify the roles of two different endothelin (ET) receptors in the pulmonary vasculature, the localization and distribution of endothelin-A (ETA) and ETB receptors were investigated in rat lung under normal and hypoxic conditions by an immunohistochemical method. We also carried out in situ hybridization for ETB receptor. In normal rats, ETA receptor is localized in the media of the pulmonary artery and vein with predominant distribution in such proximal segments as elastic arteries and large muscular arteries. ETB receptor is expressed in the intima and media of pulmonary vessels. The distribution of ETB receptor in the media predominates in the distal segments of the pulmonary artery, whereas its distribution in the intima is greater in the proximal segments. Immunoreactivity for ETA receptor increases in the media of the distal segments of the pulmonary artery after exposure to hypobaric hypoxia. Semiquantitative evaluation showed immunoreactivity for ETA receptor in the pulmonary arteries accompanying the terminal bronchioles, respiratory bronchioles, and alveolar ducts to be increased by 2.5-, 5-, and 20-fold after 14 d exposure to hypoxia, respectively. The messenger RNA and immunoreactivity for ETB receptor increased significantly in the intima of the distal segments of pulmonary artery after 7 and 14 d exposure to hypoxia. These results suggest that the vasoconstrictive effects of ET-1 are exerted mainly through ETA receptor in the proximal segments of the pulmonary artery and vein, whereas its effects in the distal segments are mediated by ETA and ETB receptors in normal rats. ETA receptors that increase in resistance arteries after exposure to hypoxia appear to play an important role in the vascular remodeling associated with hypoxic pulmonary hypertension. Because ETB receptors in the endothelium mediate ET-1-induced vasodilatory effects, the increase in endothelial ETB receptors may counteract the development of hypoxic pulmonary hypertension.