  Using ROSAT observations, we estimate gas pressures in the X-ray-emitting
medium surrounding 63 FRII radio galaxies and quasars. We compare these
pressures with the internal pressures of the radio-emitting plasma estimated by
assuming minimum energy or equipartition. In the majority of cases (including
12/13 sources with modelled, spatially resolved X-ray emission) radio sources
appear to be underpressured with respect to the external medium, suggesting
that simple minimum-energy arguments underestimate the sources' internal energy
density. We discuss possible departures from the minimum energy condition and
the consequences of our result for models of the dynamics of radio galaxies, in
particular self-similar models (Kaiser & Alexander 1997).
