  We present calculations of the spectral and temporal radiative signatures
expected from ultrarelativistic protons in compact sources. The coupling
between the protons and the leptonic component is assumed to occur via
Bethe-Heitler pair production. This process is treated by modeling the results
of Monte-Carlo simulations and incorporating them in a time-dependent kinetic
equation, that we subsequently solve numerically. Thus, the present work is, in
many respects, an extension of the leptonic `one-zone' models to include
hadrons. Several examples of astrophysical importance are presented, such as
the signature resulting from the cooling of relativistic protons on an external
black-body field and that of their cooling in the presence of radiation from
injected electrons. We also investigate and refine the threshold conditions for
the 'Pair Production/Synchrotron' feedback loop which operates when
relativistic protons cool efficiently on the synchrotron radiation of the
internally produced Bethe-Heitler pairs. We demonstrate that an additional
component of injected electrons lowers the threshold for this instability.
