  We examine the gas and stellar metallicities in a sample of HII galaxies from
the Sloan Digital Sky Survey, which possibly contains the largest homogeneous
sample of HII galaxy spectra to date. We eliminated all spectra with an
insufficient signal-to-noise ratio, without strong emission lines, and without
the [OII] lambda3727 {\AA} line, which is necessary for the determination of
the gas metallicity. This excludes galaxies with redshift <~ 0.033. Our final
sample contains ~700 spectra of HII galaxies. Through emission line strength
calibrations and a detailed stellar population analysis employing evolutionary
stellar synthesis methods, which we already used in previous works, we
determined the metallicities of both the gas and the stellar content of these
galaxies. We find that in HII galaxies up to stellar masses of 5\cdot10^9
M_sol, enrichment mechanisms do not vary with galactic mass, being the same for
low- and high-mass galaxies on average. They do seem to present a greater
variety at the high-mass end, though, indicating a more complex assembly
history for high-mass galaxies. In around 23 per cent of our HII galaxies we
find a metallicity decrease over the last few Gyr. Our results favour galaxy
evolution models featuring constantly infalling low-metallicity clouds that
retain part of the galactic winds. Above 5\cdot10^9 M_sol stellar mass, the
retention of high metallicity gas by the galaxies' gravitational potential
dominates.
