  We report results from our monitoring of the BL Lac object Mrk 421 in the
near-IR $J$ band. The observations, aimed at studying the intraday variability
(IDV) of the object, were carried out systematically over an extended (and
near-continuous) period of eight nights from the 1.2m Mount Abu Infrared
Telescope, India. There are limited studies for Mrk 421 in the $J$ band for
such an extended period. The observation epoch for this study (25 February - 5
March 2003) was chosen to significantly overlap other concurrent studies of Mrk
421 in the X-ray/$\gamma$-ray regions being conducted using the Rossi X-ray
timing explorer (RXTE) and the solar tower atmospheric Cherenkov effect
experiment (STACEE). Hence these results could be useful for a multi-wavelength
analysis of the variability behavior of Mrk 421. We find that Mrk 421 was quite
active during the observed period and showed significant IDV and short term
variability. A maximum variation of 0.89 magnitudes is seen over the entirety
of the observed period. Flaring activity, with typical brightness variations of
$\sim 0.4$, are also seen on several occasions. The extent of the variability
observed by us is compared with the results of other similar studies of Mrk 421
in the $J$ band.
