  We present high resolution spectra (0.7 A/pix) of the GPS source 4C 12.50
with large spectral coverage (~ 4500A) taken with the 4.2m William Herschel
Telescope on La Palma. The slit was aligned along PA 160 to include the nucleus
and emission line region to the NW. An asymmetric halo extending 20 kpc NW and
12 kpc SE from the nucleus is clearly seen. At the position of the nucleus we
observe unusually broad forbidden emission line components (broadest component:
FWHM ~ 2000 km s^-1), blue shifted by up to 2000 km s^-1 with respect to the
halo of the galaxy and HI absorption. We interpret this as material in outflow.
We measure E(B-V) = 1.44 for the broadest, most kinematically disturbed
component, corresponding to an actual Hbeta flux 130 times brighter than that
measured. We calculate an upper limit for the mass of the line emitting gas of
order 10^6 solar masses for both the intermediate and broad components. Our
results are consistent with 4C 12.50 being a young radio source.
