  We have re-evaluated empirical expressions for the abundance determination of
N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and
constructing an appropriate grid of photoionization models with state-of-the
art model atmospheres. Using these expressions we have derived heavy element
abundances in the $\sim$ 310 emission-line galaxies from the Data Release 3 of
the Sloan Digital Sky Survey (SDSS)with an observed Hbeta flux F(Hbeta)> 1E-14
erg s^{-1} cm^{-2} and for which the [O III] 4363 emission line was detected at
least at a 2sigma level, allowing abundance determination by direct methods.
The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ~
7.1 (Zsun/30) to 8.5 (0.7 Zsun). The SDSS sample is merged with a sample of 109
blue compact dwarf (BCD) galaxies with high quality spectra, which contains
extremely low-metallicity objects. We use the merged sample to study the
abundance patterns of low-metallicity emission-line galaxies. We find that
extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Zsun/12) are rare
in the SDSS sample. The alpha element-to-oxygen abundance ratios do not show
any significant trends with oxygen abundance, in agreement with previous
studies, except for a slight increase of Ne/O with increasing metallicity,
which we interpret as due to a moderate depletion of O onto grains in the most
metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value,
by up to 1 dex at the high metallicity end. (abridged)
