  We present sodium abundances for a sample of nearby stars. All results have
been derived from NLTE statistical equilibrium calculations. The influence of
collisional interactions with electrons and hydrogen atoms is evaluated by
comparison of the solar spectrum with very precise fits to the Na I line cores.
The NLTE effects are more pronounced in metal-poor stars since the statistical
equilibrium is dominated by collisions of which at least the electronic
component is substantially reduced. The resulting influence on the
determination of sodium abundances is in a direction opposite to that found
previously for Mg and Al. The NLTE corrections are about -0.1 in thick-disk
stars with [Fe/H] about -0.6. Our [Na/Fe] abundance ratios are about solar for
thick- and thin-disk stars. The increase in [Na/Fe] as a function of [Fe/H] for
metal-rich stars found by Edvardsson et al. (1993) is confirmed. Our results
suggest that sodium yields increase with the metallicity, and quite large
amounts of sodium may be produced by AGB stars. We find that [Na/Fe]ratios,
together with either [Mg/Fe] ratio, kinematic data or stellar evolutionary
ages, make possible the individual discrimination between thin- and thick-disk
membership.
