  We analyse the oxygen abundance and specific star formation rates (sSFR)
variations with redshift in star-forming SDSS galaxies of different masses. We
find that the maximum value of the sSFR, sSFRmax, decreases when the stellar
mass, Ms, of a galaxy increases, and decreases with decreasing of redshift. The
sSFRmax can exceed the time-averaged sSFR by about an order of magnitude for
massive galaxies. The metallicity - redshift relations for subsamples of
galaxies with sSFR = sSFRmax and with sSFR = 0.1sSFRmax coincide for massive
(log(Ms/Mo) > 10.5, with stellar mass Ms in solar units) galaxies and differ
for low-mass galaxies. This suggests that there is no correlation between
oxygen abundance and sSFR in massive galaxies and that the oxygen abundance
correlates with the sSFR in low-mass galaxies. We find evidence in favour of
that the irregular galaxies show, on average, higher sSFR and lower oxygen
abundances than the spiral galaxies of similar masses and that the mass -
metallicity relation for spiral galaxies differs slightly from that for
irregular galaxies. The fact that our sample of low-mass galaxies is the
mixture of spiral and irregular galaxies can be responsible for the dependence
of the metallicity - redshift relation on the sSFR observed for the low-mass
SDSS galaxies. The mass - metallicity and luminosity - metallicity relations
obtained for irregular SDSS galaxies agree with corresponding relations for
nearby irregular galaxies with direct abundance determinations. We find that
the aperture effect does not make a significant contribution to the redshift
variation of oxygen abundances in SDSS galaxies.
