XMM-Newton observations of Markarian 421

The BL Lac object Mrk 421 was observed on May 25, 2000 during the XMM-Newto n Cal/PV phase. The high throughput of the X-ray telescopes and the spectra l capabilities of the instruments allow an uninterrupted temporal and spect ral study of the source with unprecedented time resolution. Mrk 421 was fou nd at a relatively high state with a 2-6 keV flux of (1.3-1.9) 10(-10) erg cm(-2) s(-1). The observed intensity variations by more than a factor of th ree at highest X-ray energies are accompanied by complex spectral variation s with only a small time lag (tau = 265(-102)(+116) s) between the hard and soft photons. The (0.2-10) keV spectrum can be well fitted by a broken pow er law and no absorption structures are found in the source spectrum at the high spectral resolution of the transmission gratings.