  Metallicity gradients derived from planetary nebulae (PNe) using O, Ne, and
Ar abundances are studied and compared to those from HII regions in the
galaxies M31, M33, NGC 300 and the Milky Way. Galactocentric radii and chemical
abundances were collected from the literature, carefully selecting a
homogeneous sample for each galaxy. Metallicity gradients shown by PNe are
flatter than those of HII regions in all cases. The extreme case is M31 where
PN abundances are not related to galactocentric distances and the gradients are
consistent with zero. To analyze the evolution of gradients with time we build
gradients for Peimbert Type I and non-Type I PNe finding that Type I PNe show
steeper gradients than non-Type I PNe and more similar to the ones of HII
regions indicating that the chemical gradients might steepen with time.
Alternatively, the flat gradients for old PNe show that radial migration could
have an important role in the evolution of galaxies.
