  We present measurements of radial velocities for the narrow-line region gas
(NLR) in the Seyfert 2 galaxy Mrk 3 out to ~1 kpc from the nucleus. The
observations consist of two datasets, both using the Space Telescope Imaging
Spectrograph on board the Hubble Space Telescope: 1) An [O III] slitless
spectrum with the G430M grating of the inner 3" around the nucleus, and 2) a
long-slit observation centered on the nucleus (PA = 71 deg) using the G430L
grating and the 52" x 0".1 aperture. Our results produce radial velocity maps
of the emission-line gas. These maps indicate general trends in the gas motion,
which include: blueshifts and redshifts on either side of the nucleus, steep
velocity rises from systemic up to ~ +/-700 km/s taking place in the inner 0".3
(0.8 kpc) both east and west of the nucleus, gradual velocity descents back to
near-systemic values from 0".3-1".0, slightly uneven velocity amplitudes on
each side of the nucleus, and narrow velocity ranges over the entire observed
region. When fitted to kinematic modeling programs for the NLR gas, the data
clearly favor a model where the gas exists in a partially filled bicone, is
accelerated radially away from the nucleus, and is followed by a constant
deceleration. This geometry and general kinematic model is in agreement with
previous work done on the NLR gas of NGC 1068 and NGC 4151. On scales of
hundreds of parsecs, we conclude that radial outflow may be a common feature of
Seyfert galaxies.
