Nitric oxide (NO) has been shown to down regulate its own synthesis in vitro. We tested the hypothesis that NO inhalation (30 ppm under normoxic conditions) could decrease the release of endogenous endothelial NO, and thus alter pulmonary vasoreactivity. Pulmonary vasoreactivity was assessed in isolated perfused rat lungs immediately or 6 h after a 48 h NO inhalation period, and compared with a control group. NO inhalation resulted in an increase in pulmonary vasoconstrictor reactivity to angiotensine II and U-46619, a reduction in the potentiation by the eNOS inhibitor L-NAME of the angiotensine II response, a decrease in endothelium-dependent vasodilation to arginine vasopressin, whereas non-endothelium-dependent vasodilation to sodium nitroprusside remained unaltered. These alterations returned to control values in the group studied 6 h after the end of NO inhalation, and were not prevented by inhibition of the prostanoid synthesis, or by pretreatment with the endothelin receptors antagonist Bosentan. These results indicate that NO inhalation over 2 d induces a reversible alteration of pulmonary vasoreactivity in relationship with a decrease in endogenous NO release. Inhibition of eNOS could be involved.