  I examine how large violation of CP and T is allowed in long base line
neutrino experiments. When we attribute both the atmospheric neutrino anomaly
and the solar neutrino deficit to neutrino oscillation we may have a sizable T
violation effect proportional to the ratio of two mass differences; it is
difficult to see CP violation since we can't ignore the matter effect. I give a
simple expression for T violation in the presence of matter.
