  A general exact analytic expression for the probability of matter-enhanced
two-neutrino transitions in a medium (MSW, RSFP, generated by neutrino FCNC
interactions, etc.) is derived. The probability is expressed in terms of three
real functions of the parameters of the transitions: the ``jump'' probability
and two phases (angles). The results obtained can be utilized, in particular,
in the studies of the matter-enhanced transitions/conversions of solar and
supernova neutrinos. An interesting similarity between the Schroedinger
equation for the radial part of the non-relativistic wave function of the
hydrogen atom and the equation governing the MSW transitions of solar neutrinos
in the exponentially varying matter density in the Sun is also briefly
discussed.
