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 ] \sim - 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 .
( [ ] ) 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 .