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For the high-frequency peaked BL Lac object Mrk 421 we study the variation ofthe spectral energy distribution (SED) as a function of source activity, fromquiescent to active. We use a fully automatized chi-squared minimizationprocedure, instead of the "eyeball" procedure more commonly used in theliterature, to model nine SED datasets with a one-zone Synchrotron-Self-Compton(SSC) model and examine how the model parameters vary with source activity. Thelatter 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 sourceactivity, whereas the electron spectrum's break energy and the Doppler factorincrease -- the other SSC parameters turn out to be uncorrelated with sourceactivity. In the SSC framework these results are interpreted in a picture wherethe synchrotron power and peak frequency remain constant with varying sourceactivity, through a combination of decreasing magnetic field and increasingnumber density of electrons below the break energy: since this leads to anincreased electron-photon scattering efficiency, the resulting Compton powerincreases, and so does the total (= synchrotron plus Compton) emission.

The environment and distribution of emitting electrons as a function of source activity in Markarian 421