  Blazars are characterized by large amplitude and fast variability, indicating
that the electron distribution is rapidly changing, often on time scales
shorter than the light crossing time. We study the time dependent behavior of
the electron distribution after episodic electron injection phases, and
calculate the observed synchrotron and self Compton radiation spectra. Since
photons produced in different part of the source have different travel times,
the observed spectrum is produced by the electron distribution at different
stages of evolution. Time delays between the light curves of fluxes at
different frequencies are possible, as illustrated for the specific case of the
BL Lac object Mkn 421.
