  We investigate the limitations on the thermal description of three
dimensional BTZ black holes. We derive on physical grounds three basic mass
scales that are relevant to characterize these limitations. The Planck mass in
2+1 dimensions indicate the limits where the black hole can emit Hawking's
radiation. We show that the back reaction is meaningless for spinless BTZ black
hole. For stationary BTZ black holes the nearly extreme case is analyzed
showing that may occur a break down of its description as a thermal object.
