  I numerically analyze Joule heating in large Bi2212 mesas while taking into
account typical thermal conductivities and their temperature dependencies of
all the materials involved in the heat dissipation and its removal. Such mesas
are used in experiments on THz-range radiation. The analysis shows that the
temperature increases with bias current and is distributed unevenly along the
mesas. The temperature of the mesa's middle part can even exceed $T_c$ at
sufficiently high bias. The non-uniform temperature distribution can possibly
be important for synchronization of emission from different junctions in the
mesas. The overall current-voltage characteristics are also calculated
self-consistently showing a negative differential conductance in a wide range
of currents.
