  We explore the possibility to calibrate massive cluster ellipticals as
cosmological standard rods using the Fundamental Plane relation combined with a
correction for luminosity evolution. Though cluster ellipticals certainly
formed in a complex way, their passive evolution out to redshifts of about 1
indicates that basically all major merging and accretion events took place at
higher redshifts. Therefore, a calibration of their luminosity evolution can be
attempted. We propose to use the Mg$-\sigma$ relation for that purpose because
it is independent of distance and cosmology. We discuss a variety of possible
caveats, ranging from dynamical evolution to uncertainties in stellar
population models and evolution corrections to the presence of age spread.
Sources of major random and systematic errors are analysed as well. We apply
the described procedure to nine elliptical galaxies in two clusters at
$z=0.375$ and derive constraints on the cosmological model. For the best
fitting $\Lambda$-free cosmological model we obtain: $q_o \approx 0.1$, with
90% confidence limits being $0 < q_o < 0.7$ (the lower limit being due to the
presence of matter in the Universe). If the inflationary scenario applies (i.e.
the Universe has flat geometry), then, for the best fitting model, matter and
$\Lambda$ contribute about equally to the critical cosmic density (i.e.
$\Omega_m \approx \Omega_\Lambda \approx 0.5$). With 90% confidence
$\Omega_\Lambda$ should be smaller than 0.9.
