************************************************************************* 53020 Recent ROSAT and ASCA observations of a sizeable sample of high- redshift radio-loud quasars have revealed that substantial intrinsic absorption is a common property suggesting a different beahviour among low and high redshift objects. On the other side, the spectral information available for radio-quiet quasars is almost absent. The proposed observations of 2 high-redshift radio-quiet quasars will allow us to obtain broad band (1-30 keV in the source rest frame) X-ray spectra of good statistical quality. The comparison with the available data for low redshift objects will provide new constraints on the quasar emission mechanism and evolutive properties. This proposal follows a previous succesful one (AO-4) and is part of a more general program we have in common with our RIKEN colleagues. *************************************************************************