  (Abridged) We present in this paper a detailed analysis of the effect of
environment on the star-formation activity of galaxies within the EDR of the
SDSS. We have used the Halpha emission line to derive the star-formation rate
(SFR) for each galaxy within a volume-limited sample of 8598 galaxies with 0.05
< z < 0.095 and M(r)<= -20.45. We find that the SFR of galaxies is strongly
correlated with the local (projected) galaxy density and thus we present here
the density-SFR relation that is analogous to the density-morphology relation.
The effect of density on the SFR of galaxies is seen in three ways. First, the
overall distribution of SFRs is shifted to lower values in dense environments
compared with the field population. Second, the effect is most noticeable for
the strongly star-forming galaxies in the 75th percentile of the SFR
distribution. Third, there is a ``break'' (or characteristic density) in the
density-SFR relation at a local galaxy density of 1h-2 Mpc-2. To understand
this break further, we have studied the SFR of galaxies as a function of
clustercentric radius from 17 clusters and groups objectively selected from the
SDSS EDR data. The distribution of SFRs of cluster galaxies begins to change,
compared with the field population, at a clustercentric radius of 3-4 virial
radii, which is consistent with the characteristic break in density that we
observe in the density-SFR relation. Our tests suggest that the
density-morphology relation alone is unlikely to explain the density-SFR
relation we observe. Taken all together, these works demonstrate that the
decrease in SFR of galaxies in dense environments is a universal phenomenon
over a wide range in density (from 0.08 to 10h-2 Mpc-2) and redshift (out to z
= 0.5).
