  We study the matter effects for solar neutrino oscillations in a general
scheme, without any constraint on the number of sterile neutrinos and the
mixing matrix elements, only assuming a realistic hierarchy of neutrino
squared-mass differences in which the smallest squared-mass difference is
effective in solar neutrino oscillations. The validity of the analytic results
are illustrated with a numerical solution of the evolution equation in the
simplest case of four-neutrino mixing with the realistic matter density profile
inside the Sun.
