  We derive a relativistic expression for neutrino energy losses caused by the
direct Urca processes in degenerate baryon matter of neutron stars. We use two
different ways to calculate the emissivity caused by the reactions to our
interest. First we perform a standard calculation by Fermi's ''golden'' rule.
The second calculation, resulting in the same expression, is performed with the
aid of polarization functions of the medium. Our result for neutrino energy
losses strongly differs from previous non-relativistic results. We also discuss
nonconservation of the baryon vector current in reactions through weak charged
currents in the medium, when the asymmetry between protons and neutrons is
considered. The above effects, not discussed in the literature before,
substantially modify the polarization functions responsible for the induced
weak charged currents in baryon matter.
