Nitric oxide (NO) synthase inhibitors potentiate hypoxic vasoconstriction (HV), suggesting that NO production during hypoxia normally acts to attenuate HV. To begin to examine the effect of hypoxia on lung NO production, we studied four groups of isolated neonatal pig lungs. In three groups of lungs, the accumulation of nitrite/nitrate (NOx-) was measured in the recirculating perfusate during ventilation with a control gas mixture (Cont), a hypoxic gas mixture (Hyp), or the control gas mixture with N omega-nitro-L-arginine methyl ester (L-NAME) added to the perfusate. Both hypoxia and L-NAME significantly increased perfusion pressure [pulmonary arterial pressure (Pa)-pulmonary venous pressure (Pv)] compared with control. NOx- accumulated in the perfusate at an average rate of 9.1 +/- 2.3 (SE) nmol/min in Cont, 3.7 +/- 0.8 nmol/min (P < 0.05 vs. control) in Hyp, and 3.7 +/- 0.6 nmol/min (P < 0.05 vs. control) in L-NAME. In the fourth group of lungs, exhaled NO output was measured during ventilation with the control gas mixture, the hypoxic gas mixture, and the control gas mixture with L-NAME added to the perfusate. Pa-Pv increased significantly with both hypoxia and L-NAME in these lungs. The exhaled NO output also decreased significantly with both hypoxia and L-NAME. These results suggest that in this preparation there was continuous production of NO that was decreased by hypoxia or L-NAME. It is not clear how the potentiation of HV by NO inhibitors and inhibition of NO production by hypoxia are linked.