  The results of a BeppoSAX TOO observation of the BL Lac object Mkn421 during
a high intensity state are reported and compared with monitoring X-ray data
collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky
Monitor(ASM). The 0.1-100 keV spectrum of Mkn421 shows continuous convex
curvature that can be interpreted as the high-energy end of the synchrotron
emission. The source shows significant short-term temporal and spectral
variability, which can be interpreted in terms of synchrotron cooling. The
comparison of our results with those of previous observations when the source
was a factor 3-5 fainter shows evidence for strong spectral variability, with
the maximum of the synchrotron power shifting to higher energy during high
states. This behaviour suggest an increase in the number of energetic electrons
during high states.
