  New results on the time variation of the radial abundance gradients in the
galactic disk are presented on the basis of four different samples of planetary
nebulae. These comprise both smaller, homogeneous sets of data, and larger but
non-homogeneous samples. Four different chemical elements are considered,
namely, O, S, Ar, and Ne. Other objects such as open clusters, cepheids and HII
regions are also taken into account. Our analysis support our earlier
conclusions in the sense that, on the average, the radial abundance gradients
have flattened out during the last 6 to 8 Gyr, with important consequences for
models of the chemical evolution of the Galaxy.
