  Recent work on abundance gradients have focussed not only on their
magnitudes, but also on their spatial and temporal variations. In this work, we
analyze the behaviour of radial abundance gradients in the galactic disk giving
special emphasis on these variations. The data used includes planetary nebulae
and objects in different age brackets, namely open clusters, HII regions,
cepheid variables and stars in OB associations. We find evidences for a space
variation of the radial gradients as measured for element ratios such as O/H,
S/H, Ne/H, Ar/H and [Fe/H], in the sense that the gradients tend to flatten out
at large galactocentric distances. Moreover, near the bulge-disk interface a
steep decrease in the abundances is observed. The time evolution of the
gradients is also evaluated on the basis of approximate ages attributed to the
central stars of planetary nebulae and open cluster stars. It is concluded that
the available data is consistent with a time flattening of the gradients during
the last 6 to 8 Gyr, a time interval in which the age determinations are
probably more accurate.
