  The quasi-symmetric shape of the lightcurves, as seen in the X-rays and in
the optical, together with the fast variations of the flux observed, suggest
that the cooling time for the highest energy electrons may be shorter than the
light crossing time $R/c$. Moreover the existence of time lags between
lightcurves at different frequencies can be interpreted, in the frame of
homogeneous models, as due to the different cooling times of electrons of
different energy. To reproduce in detail the variability pattern at different
wavelengths we need to study the time dependent behavior of the emitting
particle distribution and to take into account of the different light travel
times of photons produced in different regions of the source. We apply the
model to PKS 2155-304 (1991 and 1994 campaigns).
