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 .