We show in a simple and general way that matter effects do not contrib
ute to the average value of the transition probabilities of solar nu(e
)'s into other states in the case of maximal mixing of any number of m
assive neutrinos. We also show that future solar neutrino experiments
(Super-Kamiokande and SNO) will allow to test the model with maximal m
ixing of three massive neutrinos in a way that does not depend on the
initial solar neutrino flux.