  We investigate the dissipation of the bulk kinetic energy of a relativistic
jet at different distances from the central power--house and analyse in detail
how the dissipated energy is radiated away. We assume that the location of the
dissipation region is a function of the bulk Lorentz factor Gamma of the jet,
being closer to the center for smaller Gamma. This assumption is naturally
fulfilled in the internal shock scenario. The dissipated energy is partially
used to accelerate electrons and to amplify the magnetic field. This process
creates a source inside the jet (blob). Such blobs may efficiently produce
synchrotron and inverse Compton emission. We find that even if the blobs or
shells responsible for the blazar activity carry the same energy (in bulk
kinetic form), the fact that they move at different Gamma can produce dramatic
variations in different bands, even if the bolometric luminosity is instead
very similar. This is due to the relative importance of the synchrotron,
self-Compton and external Compton radiation processes, which greatly changes by
changing Gamma and the compactness of the source, even if the total radiated
energy is constant. We then find that the jet can produce most of its radiative
output at small distances from the putative black-hole and its accretion disk,
if this implies a low level of emitted MeV-GeV flux. Our findings, which we
apply for illustrative purposes to the blazar 3C 454.3, will be easily testable
by the coming gamma-ray satellite, such as AGILE and GLAST.
