  We show that neutrino oscillations in matter are always accompanied by
collective forces on the medium. This effect may produce interesting
consequences for the background and the neutrino oscillations themselves. The
force is maximal in the case of resonant neutrino conversion in the adiabatic
regime. We study here the forces driven by $\nu_e-\nu_{\mu,\tau}$ and
$\nu_e-\nu_s$ MSW conversion and shortly discuss their possible relevance for
the dynamics of a type II supernova.
