  We present the results of an extensive Fabry-Perot investigation of the
extended line-emission gas surrounding powerful radio galaxies. High spatial
(1.4 arcsec) and kinematical (400 km/s) resolution observations of the
[OII]3727 emission-line of the powerful, double radio source 3C34 (z=0.689) are
given. We present evidence that anisotropic radiation emanating from a hidden
AGN is responsible for photoionizing a line-emission region extending more than
120 kpc. This is indicated by the distinctively bi-conical morphology of 3C34's
[OII] gas. A number of line-emission components may also show various degrees
of `shadowing' outward from the central ionizing source. A simple
photoionization model is used to show that this interpretation is energetically
viable on these length scales. The luminosity of the hidden central AGN,
necessary to account for the observed [OII] luminosity, is compatible with that
of a typical 3CR quasar at a similar redshift. Although this interpretation can
account for the excitation and most of the unique shape of the warm gas, it is
insufficient to explain the velocity and line-width structures observed with
our Fabry-Perot interferometer. Therefore, we also propose that the illuminated
medium surrounding 3C34 is the result of gas swept aside by the lateral
expansion and backflow of the radio source lobes.
