  The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.
