  We present spectroscopic and photometric observations of the luminous narrow-
line Type IIP (plateau) supernova 1994W. After the plateau phase (t >120 days),
the light curve dropped by 3.5 mag in V in only 12 days. Between 125 and 197
days after explosion the supernova faded substantially faster than the decay
rate of Co-56, and by day 197 it was 3.6 magnitudes less luminous in R compared
to SN 1987A. The low R-luminosity could indicate less than 0.0026 {+0.0017}/
{-0.0011} Msun of Ni-56 ejected at the explosion, but the emission between 125
and 197 days must then have been dominated by an additional power source, pre-
sumably circumstellar interaction. Alternatively, the late light curve was
dominated by Co-56 decay. In this case, the mass of the ejected Ni-56 was 0.015
{+0.012}/{-0.008} Msun, and the rapid fading between 125 and 197 days was most
likely due to dust formation. Though this value of the mass is higher than in
the case with the additional power source, it is still lower than estimated for
any previous Type II supernova. Only progenitors with M(ZAMS) = 8-10 Msun and
M(ZAMS) > 25 Msun are expected to eject such low masses of Ni-56. If M(ZAMS) =
8-10 Msun, the plateau phase indicates a low explosion energy, while for a
progenitor with M(ZAMS) > 25 Msun the energy can be the canonical 1.0E{51}
ergs. As SN 1994W was unusually luminous, the low-mass explosion may require an
uncomfortably high efficiency in converting explosion energy into radiation.
This favors a M(ZAMS) > 25 Msun progenitor. The supernova's narrow (roughly
1000 km s^{-1}) emission lines were excited by the hot supernova spectrum,
rather than a circumstellar shock. The thin shell from which the lines origi-
nated was most likely accelerated by the radiation from the supernova.
