  The temporal behaviour of the radial abundance gradients has important
consequences for models of the chemical evolution of the Galaxy. We present a
comparison of the time variation of the abundance gradients in the Milky Way
disk as determined from a sample of planetary nebulae, open clusters, cepheids
and young objects, such as stars in OB associations and HII regions. We
conclude that the [Fe/H] gradients as measured in open cluster stars strongly
support the time flattening of the abundance gradient as determined from O/H
and S/H measurements in planetary nebulae. This conclusion is also supported by
the cepheid variables, for which very accurate gradients and ages can be
determined, and also by some recent estimates for OB stars and HII regions. It
is estimated that the average flattening rate for the last 8 Gyr is in the
range 0.005-0.010 dex/kpc/Gyr.
