  The gamma-ray emission above 250 GeV from the BL Lac object Markarian 421 was
observed by the CAT Cherenkov imaging telescope between December, 1996, and
June, 2000. In 1998, the source produced a series of small flares, making it
the second extragalactic source detected by CAT. The time-averaged differential
spectrum has been measured from 0.3 to 5 TeV, which is well fitted with a power
law with an index of -2.88+-0.12(stat)+-0.06(syst). In 2000, the source showed
an unprecedented activity, with variability time-scales as short as one hour,
as for instance observed during the night between 4 and 5 February. The 2000
time-averaged spectrum measured is compatible with that of 1998, but some
indication of a spectral curvature is found between 0.3 and 5 TeV. The
possibility of TeV spectral hardening during flares is also discussed, and the
results are compared to those obtained on the other TeV BL Lac, Markarian 501.
