  In this paper we apply the fibering method of Pohozaev to a
Schr\"odinger-Poisson system, with prescribed $L^{2}$ norm of the unknown, in
the whole $\mathbb R^{3}$. The method makes clear the role played by the
exponents $p=3, p=8/3, p=10/3$.
  Beside to show as old results can be obtained in a unified way, we exhibit
also new ones.
