To examine the effect of chronically elevated CO₂ on excitability and function of neurons, we exposed mice to 8% and 12% CO₂ for 4 weeks (starting at 2 days of age), and examined the properties of freshly dissociated hippocampal neurons obtained from slices. Chronic CO₂-treated neurons (CC) had a similar input resistance (R_m) and resting membrane potential (V_m) as control (CON). While treatment with 8% CO₂ did not change the rheobase (64±11 pA, n=9 vs. 47±12 pA, n=8 for CC 8% vs. CON), 12% CO₂ treatment increased it significantly (73±8 pA, n=9, p=0.05). Furthermore, the 12% CO₂ but not the 8% CO₂ treatment decreased the Na⁺ channel current density (244±36 pA/pF, n=17, vs. 436±56 pA/pF, n=18, for CC vs. CON, p=0.005). Recovery from inactivation was also lowered by 12% but not 8% CO₂. Other gating properties of Na⁺ current, such as voltage-conductance curve, steady state inactivation, and time constant for deactivation, were not modified by either treatment. Western blot analysis showed that the expression of Na⁺ channel types I, II and III was not changed by 8% CO₂ treatment but their expression was significantly decreased by 20-30% (p=0.03) by the 12% treatment. We conclude from these data and others (Gu et al. 2004) that 1) neuronal excitability and Na⁺ channel expression depend on the duration and level of CO₂ exposure and 2) maturational changes occur in early life regarding neuronal responsiveness to CO₂. (This work was supported by NIH grants PO1 HD-32573, RO1 NS-35918 and RO1 HL-66327).