To study the effect of chronically elevated CO₂ on excitability and function of neurons, we exposed mice to 7.5-8% CO₂ for ~2 weeks (starting at 2 days of age), and examined the properties of freshly dissociated hippocampal neurons. Neurons from control mice(CON) and those which were chronically exposed to elevated CO₂ had similar resting membrane potential (V_m) and input resistance (R_m). Exposed neurons, however, had a lower rheobase and a higher Na⁺ current density (580±73 pA/pF, n=27) than CON neurons (280±51 pA/pF, n=34, P<0.01). In addition, the conductance-voltage curve was shifted in a more negative direction in exposed than in CON (mid-point of the curve -46±3 mV for exposed and -34±3 mV for CON, P<0.01), while the steady state inactivation curve was shifted in a more positive direction in exposed than in CON (mid-point of the curve -59±2 mV for exposed and -68±3 mV, for CON, P<0.01). The time constant for deactivation (_d) at -100 mV was much smaller in exposed than in CON neurons(0.8±0.1 ms for exposed and 1.9±0.3 ms for CON, P<0.01). Immunoblotting for Na⁺ channel protein (subtypes I, II, III) was performed on the hippocampus. Our data indicate that Na⁺ channel subtype I was significantly increased (43%, n=4, P<0.05) rather than II or III in the hippocampus of CO₂-exposed mice. We conclude that in mice exposed to elevated CO₂, a) the increased neuronal excitability is due to alterations in Na⁺ current and Na⁺ channel characteristics, and b) the up-regulation of Na⁺ channel subtype I contributes, at least in part, to the increase in Na⁺ current density.