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We analyse the oxygen abundance and specific star formation rates (sSFR)variations with redshift in star-forming SDSS galaxies of different masses. Wefind that the maximum value of the sSFR, sSFRmax, decreases when the stellarmass, Ms, of a galaxy increases, and decreases with decreasing of redshift. ThesSFRmax can exceed the time-averaged sSFR by about an order of magnitude formassive galaxies. The metallicity - redshift relations for subsamples ofgalaxies 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 differfor low-mass galaxies. This suggests that there is no correlation betweenoxygen abundance and sSFR in massive galaxies and that the oxygen abundancecorrelates with the sSFR in low-mass galaxies. We find evidence in favour ofthat the irregular galaxies show, on average, higher sSFR and lower oxygenabundances than the spiral galaxies of similar masses and that the mass -metallicity relation for spiral galaxies differs slightly from that forirregular galaxies. The fact that our sample of low-mass galaxies is themixture of spiral and irregular galaxies can be responsible for the dependenceof the metallicity - redshift relation on the sSFR observed for the low-massSDSS galaxies. The mass - metallicity and luminosity - metallicity relationsobtained for irregular SDSS galaxies agree with corresponding relations fornearby irregular galaxies with direct abundance determinations. We find thatthe aperture effect does not make a significant contribution to the redshiftvariation of oxygen abundances in SDSS galaxies.

The metallicity - redshift relations for emission-line SDSS galaxies: examination of the dependence on the star formation rate