  We have compared the UV line ratios of a sample of very high redshift radio
galaxies with shock and AGN photoionization models, with the goal of
determining the balance between jet-induced shocks and AGN illumination in the
extended emission line regions. We find that the UV line ratios cannot be
explained in terms of photoionization of solar abundance gas by the classical
power law of index $\alpha=$-1.5, which successfully reproduces the general
trends defined by the optical line ratios of low redshift radio galaxies. Pure
shock models also provide a poor fit to the data. However, photoionization by a
power law of index -1.0 provides an excellent fit to the UV line ratios. This
suggests that the ionizing continuum spectral shape may depend on radio
luminosity and/or redshift, such that it becomes harder as the radio power
and/or redshift increase. However, an alternative possibility is that we are
seeing the first signs of chemical evolution in these objects, since a
power-law of index -1.5 with low metallicity also provides a good fit to the
data.
  We further show that the UV line ratios provide a sensitive test of the
ionization mechanism for the lower ionization conditions prevalent in some low
redshift jet-cloud interaction candidates.
