  A realistic medium- or long-baseline neutrino experiment may suffer from
terrestrial matter effects which are likely to contaminate the genuine
T-violating asymmetry between \nu_\alpha \to \nu_\beta and \nu_\beta \to
\nu_\alpha oscillations. With the help of the commutators of lepton mass
matrices in matter, we show that this kind of contamination is negligible for a
variety of experiments provided the neutrino beam energy E and the baseline
length L satisfy the condition 10^{-7} (L/km)^2 (GeV/E) << 1. The same
observation is true for the CP-violating term of the asymmetry between
\nu_\alpha \to \nu_\beta and \bar{\nu}_\alpha \to \bar{\nu}_\beta oscillations
in matter.
