The nitric oxide (NO)-cGMP signal transduction pathway plays an important role in the regulation of pulmonary vascular tone and resistance in pulmonary hypertension. A number of studies have demonstrated that endothelial (e) and inducible nitric oxide synthases (NOS) are upregulated in hypoxia-exposed rat lung. These changes in NOS expression have been found to correlate with the process of pulmonary vascular remodeling in hypoxia-induced pulmonary hypertension, and remodeling is increased in the absence of eNOS. In this study, we examined the expression and localization of soluble guanylate cyclase (sGC), the primary receptor for NO, in hypoxia- and normoxia-treated rat lungs. Male Sprague-Dawley rats were exposed to hypoxia (10% O(2), normobaric) or normoxia for 1, 3, 5, and 21 days. The lungs were used for Western analysis of sGC protein, sGC enzyme activity, immunohistochemistry using antiserum against sGC alpha(1)- and beta(1)-subunits, and nonradioactive in situ hybridization (NRISH) using a digoxigenin-labeled sGC alpha(1)-subunit cRNA probe. Western blot analysis revealed a more than twofold increase of sGC protein alpha(1)-subunit in rat lungs exposed to 3, 5, and 21 days of hypoxia, correlating well with sGC enzyme activity. Immunohistochemistry and NRISH demonstrated increased expression of sGC in the smooth muscle cells of the pulmonary arteries and arterioles in the hypoxic rat lungs when compared with normoxic controls. Based on our results, the upregulation of sGC may play an important role in the regulation of smooth muscle tone and pressure in the pulmonary circulation during chronic hypoxia.