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Metallicity gradients derived from planetary nebulae (PNe) using O, Ne, andAr abundances are studied and compared to those from HII regions in thegalaxies M31, M33, NGC 300 and the Milky Way. Galactocentric radii and chemicalabundances were collected from the literature, carefully selecting ahomogeneous sample for each galaxy. Metallicity gradients shown by PNe areflatter than those of HII regions in all cases. The extreme case is M31 wherePN abundances are not related to galactocentric distances and the gradients areconsistent with zero. To analyze the evolution of gradients with time we buildgradients for Peimbert Type I and non-Type I PNe finding that Type I PNe showsteeper gradients than non-Type I PNe and more similar to the ones of HIIregions indicating that the chemical gradients might steepen with time.Alternatively, the flat gradients for old PNe show that radial migration couldhave an important role in the evolution of galaxies.

Metallicity gradients in M31, M33, NGC 300 and the Milky Way using abundances of different elements