  Models of relativistic jets filled with ultrarelativistic pair plasma are
very successful in explaining the broadband radiation of $\gamma$-ray blazars.
Assuming that the initial injection and cooling of ultrarelativistic pair
plasma in an AGN jet has occurred, producing the observed high-energy
$\gamma$-ray radiation, we investigate the further evolution of the pair plasma
as it continues to move out from the central engine. The effects of
thermalization and reacceleration, the emission of pair bremsstrahlung and
annihilation radiation and the bulk Compton process, and the possible
application to MeV blazars are discussed. A model calculation to the special
case of PKS~0208-512 is presented.
