  We present preliminary results on the evolution of the stellar populations of
field early-type galaxies (E/S0) from z=0.4 to z=0. The diagnostic tool used in
this study is the Fundamental Plane (FP), a tight empirical correlation between
their central velocity dispersion (sigma), effective radius (Re), and effective
surface brightness (SBe), which is observed to hold in the local Universe.
Using HST-WFPC2 archive images and spectra obtained at the ESO-3.6m telescope
we measured the FP parameters for a sample of ~30 field E/S0s at z=0.2-0.4.
Remarkably, field E/S0s at intermediate redshift also define a tight FP, with
scatter unchanged with respect to that of local samples. The intermediate
redshift FP is offset from the local one, in the sense that, for given RE and
sigma, galaxies are brighter at z=0.4 than at z=0. The implication of the
offset of the FP in terms of passive evolution of the stellar population
depends on its star formation history. In a single burst scenario, the stellar
populations of field E/S0s were formed at z=0.8-1.6 (Omega=0.3;
Omega_{Lambda}=0.7; H_0=50 km/s/Mpc). Alternatively, the bulk of stars (90% in
mass) can be formed at high redshift (z~3), and the rest in a secondary burst
occurred more recently (z~0.5-0.8).
