  The temporal behaviour of the radial abundance gradients has important
consequences on models for the chemical evolution of the Galaxy. We present a
comparison of the time variation of the abundance gradients in the Milky Way
disk as determined from a sample of planetary nebulae and open clusters. We
conclude that the [Fe/H] gradients as measured in open cluster stars strongly
support the time flattening of the abundance gradient as recently determined
from O/H measurements in planetary nebulae. We estimate the average flattening
rate in the time interval 10 Myr < t < 5 Gyr as roughly 0.005 dex kpc-1 Gyr-1.
