  We have studied the time variation of the radial abundance gradients using
samples of planetary nebulae, open clusters, cepheids and other young objects.
Based on the analysis of O/H and S/H abundances for planetary nebulae and
metallicities of the remaining objects, we concluded that the gradients have
been flattening out in the last 8 Gyr with an average rate of the order of
0.005 - 0.010 dex/(kpc Gyr). We have estimated the errors involved in the
determination of the gradients, and concluded that the existence of systematic
abundance variations is more likely than a simple statistical dispersion around
a mean value.
