No Mikheyev-Smirnov-Wolfenstein Effect in Maximal Mixing
  We investigate the possible influence of the MSW effect on the expectations for the solar neutrino experiments in the maximal mixing scenario suggested by the atmospheric neutrino data. A direct numerical calculation of matter induced effects in the Sun shows that the naive vacuum predictions are left completely undisturbed in the particular case of maximal mixing, so that the MSW effect turns out to be unobservable. We give a qualitative explanation of this result. 
P.F. Harrison, D.H. Perkins, W.G. Scott