  The time behaviour of the radial abundance gradients in the galactic disk is
investigated on the basis of four different samples of planetary nebulae,
comprising both smaller, homogeneous sets of data, and larger, albeit
non-homogeneous samples. Four different chemical elements are considered,
namely, oxygen, sulphur, argon and neon. Our analysis support our earlier
conclusions that, on the average, the radial abundance gradients have flattened
out in the last 6 to 8 Gyr.
