The Parameters of the Gamma - Ray Emission Region in Mrk 501

We determine the physical parameters (magnetic eld and Doppler factor) of the homogeneous synchrotron self-Compton model allowed by the observed X{ray to gamma-ray spectra and variability of Markarian 501 during the 15{16 April 1997 aring activity. We nd that, for 20 minute variability, a magnetic eld of 0.2 G and a Doppler factor of 20 could t these observations. We take account of photon-photon pair production interactions of gamma-ray photons occurring both inside the emission region and during propagation to Earth and nd these to be extremely important in correctly determining the allowed model parameters.