  We review the propagation of light neutrinos in matter assuming that their
mixing with heavy neutrinos is close to present experimental limits. The
phenomenological implications of the non-unitarity of the light neutrino mixing
matrix for neutrino oscillations are discussed. In particular we show that the
resonance effect in neutrino propagation in matter persists, but for slightly
modified values of the parameters and with the maximum reduced by a small
amount proportional to the mixing between light and heavy neutrinos squared.
