  Distortion of the equilibrium spectra of cosmic neutrinos due to interaction
with hotter electrons and positrons in the primeval cosmic plasma is
considered. The set of integro-differential kinetic equations for neutrinos is
accurately numerically solved. The relative corrections to neutrino energy
densities are approximately 0.9% for $\nu_e$ and 0.4% for $\nu_\mu$ and
$\nu_\tau$. This effect results in $1.4 \cdot 10^{-4}$ increase in the
primordial $^4 He$ abundance.
