Experimental pulmonary hypertension induced in a hypobaric hypoxic environment (HHE) is characterized by structural remodeling of the heart and pulmonary arteries. Endothelin-1 (ET-1), a 21-amino acid peptide, is a novel and long-lasting vasoconstrictor that increases pulmonary arterial pressure in both in vivo and in vitro experiments. To study the effects of HHE on ET-1 activity in the lungs, 59 male rats were subjected to the equivalent of an altitude of 5500 m for 1 to 4 weeks. In rats exposed to HHE, the mean pulmonary arterial pressure increased significantly from 15.2+/-0.3 (ground level) to 30.6+/-1.5 mm Hg (5500-m level) at 4 weeks, whereas their mean systemic arterial pressure remained normal. The levels of ET-1 mRNA and protein, measured respectively by Northern blot analysis and enzyme immunoassay, increased rapidly in the lungs on exposure to HHE. By in situ hybridization and immunohistochemistry, respectively, ET-1 mRNA and protein were detected in control rats in nonciliated bronchiolar epithelial cells and alveolar epithelial cells, as well as in the endothelial cells of pulmonary arteries, but minimally in the smooth muscle cells of pulmonary arteries. ET-1 mRNA- and protein-reactive smooth muscle cells in pulmonary arteries and ET-1 mRNA-reactive airway epithelial cells were significantly more abundant in rats exposed to HHE than in ground level controls. These results suggest the possibility that in smooth muscle cells in pulmonary arteries and airway epithelial cells, ET-1 may play an autocrine or paracrine role in the remodeling of blood vessels during the development of the pulmonary hypertension that is induced by HHE.