The objective of our study was to assess the role of nNOS in the ventilatory acclimatization to hypoxia. We measured the ventilation in acclimatized Bl6/CBA mice breathing 21% and 8% oxygen, used a neuronal nitric oxide synthase (nNOS) inhibitor, and assessed the expression of N-methyl-D-aspartate (NMDA) glutamate receptor and nNOS. Two groups of Bl6/CBA mice (n=60) were exposed during 2 weeks either to hypoxia (PB=420 mmHg) or normoxia (PB=760 mmHg). At the end of exposure the medulla was removed to measure the concentration of nitric oxide metabolites, the expression of NMDA-NR1 receptor and nNOS by real time RT-PCR and western blot. We also measured the ventilatory response (FIO₂=0.21 and 0.08) before and after S-methyl-L-thiocitrulline treatment (SMTC, nNOS inhibitor, 10 mg/kg, IP). Chronic hypoxia caused an increase in ventilation which was reduced after SMTC treatment mainly through a decrease in tidal volume in normoxia and in acute hypoxia. However the difference observed in the magnitude of acute hypoxic ventilarory response in acclimatized mice was not different. Acclimatization to hypoxia induced a rise in NMDA receptor as well as in nNOS and NO production. In conclusion, our study provides evidence that activation of nNOS is involved in the ventilatory acclimatization to hypoxia in mice but not in the hypoxic ventilatory response (HVR) while the increased expression of NMDA receptor expression in the medulla of chronically hypoxic mice play a role in acute HVR. These results are therefore consistent with central nervous system plasticity, partially involved in ventilatory acclimatization to hypoxia through nNOS.