  The chemical composition of the gas in galaxies versus cosmic time provides a
very important tool for understanding galaxy evolution. Although there are many
studies at high redshift, they are rather scarce at lower redshifts. However,
low redshift studies can provide important clues about the evolution of
galaxies, furnishing the required link between local and high redshift
universe. In this work we focus on the metallicity of the gas of star-forming
galaxies at low redshift, looking for signs of chemical evolution.
  To analyze the metallicity contents star-forming galaxies of similar
luminosities and masses at different redshifts. With this purpose, we present a
study of the metallicity of relatively massive (log(M_star/M_sun)>10.5) star
forming galaxies from SDSS--DR5 (Sloan Digital Sky Survey--Data Release 5),
using different redshift intervals from 0.04 to 0.4.
  We used data processed with the STARLIGHT spectral synthesis code, correcting
the fluxes for dust extinction, estimating metallicities using the R_23 method,
and segregating the samples with respect to the value of the
[NII]6583/[OII]3727 line ratio in order to break the R_23 degeneracy selecting
the upper branch. We analyze the luminosity and mass-metallicity relations, and
the effect of the Sloan fiber diameter looking for possible biases.
  By dividing our redshift samples in intervals of similar magnitude and
comparing them, significant signs of metallicity evolution are found.
Metallicity correlates inversely with redshift: from redshift 0 to 0.4 a
decrement of ~0.1 dex in 12+log(O/H) is found.
