  Open clusters offer a unique possibility to study the time evolution of the
radial metallicity gradients of several elements in our Galaxy, because they
span large intervals in age and Galactocentric distance, and both quantities
can be more accurately derived than for field stars. We re-address the issue of
the Galactic metallicity gradient and its time evolution by comparing the
empirical gradients traced by a sample of 45 open clusters with a chemical
evolution model of the Galaxy. At variance with previous similar studies, we
have collected from the literature only abundances derived from
high--resolution spectra. The clusters have distances $7 < RGC<22$ kpc and ages
from $\sim 30$ Myr to 11 Gyr. We also consider the $\alpha$-elements Si, Ca,
Ti, and the iron-peak elements Cr and Ni. The data for iron-peak and
$\alpha$-elements indicate a steep metallicity gradient for R_GC<12$ kpc and a
plateau at larger radii. The time evolution of the metallicity distribution is
characterized by a uniform increase of the metallicity at all radii, preserving
the shape of the gradient, with marginal evidence for a flattening of the
gradient with time in the radial range 7-12 kpc. Our model is able to reproduce
the main features of the metallicity gradient and its evolution with an infall
law exponentially decreasing with radius and with a collapse time scale of the
order of 8 Gyr at the solar radius. This results in a rapid collapse in the
inner regions, i.e. $R_{\rm GC}\lesssim 12$ kpc (that we associate with an
early phase of disk formation from the collapse of the halo) and in a slow
inflow of material per unit area in the outer regions at a constant rate with
time.
