  I discuss some problems connected with the high precision study of neutrino
oscillations. In the general case of $n$-neutrino mixing I derive a convenient
expression for transition probability in which only independent terms (and
mass-squared differences) enter. For three-neutrino mixing I discuss a problem
of a definition of a large (atmospheric) neutrino mass-squared difference. I
comment also possibilities to reveal the character of neutrino mass spectrum in
future reactor neutrino experiments.
