Endothelial nitric oxide (NO) synthase (eNOS) mRNA and protein and NO production are increased in hypoxia-induced hypertensive rat lungs, but it is uncertain whether eNOS gene expression and activity are increased in other forms of rat pulmonary hypertension. To investigate these questions, we measured eNOS mRNA and protein, eNOS immunohistochemical localization, perfusate NO product levels, and NO-mediated suppression of resting vascular tone in chronically hypoxic (3–4 wk at barometric pressure of 410 mmHg), monocrotaline-treated (4 wk after 60 mg/kg), and fawn-hooded (6–9 mo old) rats. eNOS mRNA levels (Northern blot) were greater in hypoxic and monocrotaline-treated lungs (130 and 125% of control lungs, respectively; P < 0.05) but not in fawn-hooded lungs. Western blotting indicated that eNOS protein levels increased to 300 ± 46% of control levels in hypoxic lungs (P < 0.05) but were decreased by 50 ± 5 and 60 ± 11%, respectively, in monocrotaline-treated and fawn-hooded lungs (P < 0.05). Immunostaining showed prominent eNOS expression in small neomuscularized arterioles in all groups, whereas perfusate NO product levels increased in chronically hypoxic lungs (3.4 ± 1.4 μM;P < 0.05) but not in either monocrotaline-treated (0.7 ± 0.3 μM) or fawn-hooded (0.45 ± 0.1 μM) lungs vs. normotensive lungs (0.12 ± 0.07 μM). All hypertensive lungs had increased baseline perfusion pressure in response to nitro-l-arginine but not to the inducible NOS inhibitor aminoguanidine. These results indicate that even though NO activity suppresses resting vascular tone in pulmonary hypertension, there are differences among the groups regarding eNOS gene expression and NO production. A better understanding of eNOS gene expression and activity in these models may provide insights into the regulation of this vasodilator system in various forms of human pulmonary hypertension.