The purpose of this study was to assess intrinsic smooth muscle mechanisms contributing to greater NO responsiveness in pulmonary vascular versus airway smooth muscle. Canine pulmonary artery (PASM) and tracheal smooth muscle (TSM) strips were used to perform concentration response studies to an NO-donor. PASM exhibited a greater NO responsiveness whether PASM and TSM were contracted with receptor agonists, phenylephrine and acetylcholine, respectively, or with KCl. The more than 10-fold difference in NO sensitivity in PASM was observed with both submaximal and maximal contractions. This difference in NO responsiveness was not due to differences in endothelial or epithelial barriers, since these were removed, nor was it due to the presence of cGMP-independent NO-mediated relaxation in either tissue. At equal concentrations of NO, the intracellular cGMP concentration ([cGMP]_i) was also greater in PASM than in TSM. Phosphodiesterase (PDE) inhibition using isobutylmethylxanthine indicated that the greater [cGMP]_i in PASM was not due to greater PDE activity in TSM. Expression of soluble guanylate cyclase (sGC) subunit mRNA (2 ± 0.2 and 1.3 ± 0.2 attomoles/µg total RNA, respectively) and protein (47.4 ± 2 and 27.8 ± 3.9 ng/mg soluble homogenate protein, respectively) was greater in PASM than in TSM. sGC₁ and sGC₁ mRNA expression were equal in PASM, but were significantly different in TSM, suggesting independent regulation of their expression. An intrinsic smooth muscle mechanism accounting for greater NO responsiveness in PASM versus TSM is greater sGC activity.