We present the results of the analysis of an extensive set of new and literature high quality data concerning Fe , C , N , O , Na , and Mg .
This analysis exploited the T _ { eff } scale determined in Gratton et al .
( 1996a ) , and the non-LTE abundance corrections computed in Gratton et al .
( 1999a ) .
Results obtained with various abundance indices are discussed and compared .
Detailed comparison with models of galactic chemical evolution will be presented in future papers of this series .

Our non-LTE analysis yields the same O abundances from both permitted and forbidden lines for stars with T _ { eff } > 4600 K , in agreement with King ( 1993 ) , but not with other studies using a lower T _ { eff } -scale for subdwarfs .
However , we obtain slightly smaller O abundances for the most luminous metal-poor field stars than for fainter stars of similar metallicities , an effect attributed to inadequacies of the adopted model atmospheres ( Kurucz 1992 , with overshooting ) for cool stars .
We find a nearly constant O overundance in metal-poor stars ( [ Fe/H ] < -0.8 ) , at a mean value of 0.46 \pm 0.02 dex ( \sigma = 0.12 , 32 stars ) , with only a gentle slope with [ Fe/H ] ( \sim - 0.1 ) ; this result is different from the steeper slope recently obtained using OH band in the near UV .

If only bonafide unmixed stars are considered , C abundances scale with Fe ones ( i.e .
[ C/Fe ] \approx 0 ) down to [ Fe/H ] \sim - 2.5 .
Due to our adoption of a different T _ { eff } scale , we do not confirm the slight C excess in the most metal poor disk dwarfs ( -0.8 < [ Fe/H ] < -0.4 ) found in previous investigations .

Na abundances scale as Fe ones in the high metallicity regime , while metal-poor stars present a Na underabundance .
None of the field stars analyzed belong to the group of O-poor and Na-rich stars observed in globular clusters .
Na is deficient with respect to Mg in halo and thick disk stars ; within these populations , Na deficiency may be a slow function of [ Mg/H ] .
Solar [ Na/Mg ] ratios are obtained for thin disk stars .