  For the high-frequency peaked BL Lac object Mrk 421 we study the variation of
the spectral energy distribution (SED) as a function of source activity, from
quiescent to active. We use a fully automatized chi-squared minimization
procedure, instead of the "eyeball" procedure more commonly used in the
literature, to model nine SED datasets with a one-zone Synchrotron-Self-Compton
(SSC) model and examine how the model parameters vary with source activity. The
latter issue can finally be addressed now, because simultaneous broad-band SEDs
(spanning from optical to VHE photon energies) have finally become available.
Our results suggest that in Mrk 421 the magnetic field decreases with source
activity, whereas the electron spectrum's break energy and the Doppler factor
increase -- the other SSC parameters turn out to be uncorrelated with source
activity. In the SSC framework these results are interpreted in a picture where
the synchrotron power and peak frequency remain constant with varying source
activity, through a combination of decreasing magnetic field and increasing
number density of electrons below the break energy: since this leads to an
increased electron-photon scattering efficiency, the resulting Compton power
increases, and so does the total (= synchrotron plus Compton) emission.
