  We present magnesium abundances derived from high resolution spectra using
several Mg I and two high excitation Mg II lines for 19 metal-poor stars with
[Fe/H] values between -1.1 and +0.2. The main goal is to search for systematic
differences in the derived abundances between the two ionisation state lines.
Our analysis shows that the one-dimensional LTE and N-LTE study finds a very
good agreement between these features. The [Mg/Fe] vs. [Fe/H] relationship
derived, despite the small sample of stars, is also in agreement with the
classical figure of increasing [Mg/Fe] with decreasing metallicity. We find a
significant scatter however, in the [Mg/Fe] ratio at [Fe/H]$\sim -0.6$ which is
currently explained as a consequence of the overlap at this metallicity of
thick and thin disk stars, which were probably formed from material with
different nucleosynthesis histories. We speculate on the possible consequences
of the agreement found between Mg I and Mg II lines on the very well known
oxygen problem in metal-poor stars. We also study the [O/Mg] ratio in the
sample stars using oxygen abundances from the literature and find that the
current observations and nucleosynthetic predictions from type II supernovae
disagree. We briefly discuss some alternatives to solve this discrepancy.
