We present results of the analysis of X-ray observations of the active M dwarfs AD Leo and EV Lac. The PSPC spectra can be fitted with one- (EV Lac) or two-component (AD Leo) isothermal mekal models, and very low metallicity (∼ 0.1 solar); during an intence flare the spectrum of EV Lac can be fitted only by adding a second component with log T ∼ 7.5. The SAX light-curves of AD Leo and EV Lac also show the occurrence of several flares. The fits of the SAX spectra require at least three thermal mekal components and best-fit coronal plasma metallicity below solar for AD Leo and only marginally below solar for EV Lac.

We have also fitted the SAX spectra of AD Leo and EV Lac with model spectra from constant cross-section static coronal loops. One-loop models fail to fit the observed spectra. A second loop component, that accounts for most of the plasma emission at high energy, is required to obtain an acceptable fit. We interpret the fit results as pointing toward the existence of various (at least two) dominant classes of coronal emitting structures: the dominant one is composed of hundreds of compact loops, with lower maximum temperature and length smaller than 0.1 the stellar radius, covering no more than 1% of stellar surface; the second one, responsible for the high energy emission, is composed at least of tens of quite elongated loops, covering a very small fraction of stellar surface.