  We analyzed the ~950 ks of spectroscopic data accumulated by the Extreme
Ultraviolet Explorer (EUVE) satellite between 1994 and 1997 for the BL Lacertae
object Markarian 421. The EUV spectrum is well detected in the 70-110 A
(112-177 eV) range and can be fitted by a power law model plus an absorption
feature in the ~71-75 A range. Previous studies of EUV absorption features in
MRK 421 and in the other EUV bright BL Lac object, PKS2155-304, explain this
absorption feature as a superposition of Doppler-smeared absorption lines
(mainly L- and M-shell transitions of Mg and Ne) originating in high-velocity
gas clouds ionized by the beamed continuum of the associated relativistic jet.
We show that, for example, Fe IX L could also be a possibility consistent with
the marginal detection of oxygen absorption lines in the X-ray range. However
physical models are highly sensitive to the assumptions on the photoionizing
continuum and the surrounding gas.
