  We investigate the electron temperature gradient in the galactic disk as
measured by young HII regions on the basis of radio recombination lines and the
corresponding gradient in planetary nebulae (PN) based on [OIII] electron
temperatures. The main goal is to investigate the time evolution of the
electron temperature gradient and of the radial abundance gradient, which is
essentially a mirror image of the temperature gradient. The recently derived
electron temperature gradient from radio recombination lines in HII regions is
compared with a new determination of the corresponding gradient from planetary
nebulae for which the progenitor star ages have been determined. The newly
derived electron temperature gradient for PN with progenitor stars with ages in
the 4-5 Gyr range is much steeper than the corresponding gradient for HII
regions. These electron temperature gradients are converted into O/H gradients
in order to make comparisons with previous estimates of the flattening rate of
the abundance gradient. It is concluded that the O/H gradient has flattened out
in the past 5 Gyr at a rate of about 0.0094 dex kpc-1 Gyr-1, in good agreement
with our previous estimates.
