  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. The emitting region is sufficiently
compact to let radiative losses dominate the cooling of high energy electrons.
We study the time dependent behaviour 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. Even a homogeneous
source then resembles an inhomogeneous one. 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.
