Hot-pressed ceramics in the PbZKh-PboFeNbOg-PbTiOg system have been shown to possess a combination of pyroelectric and dielectric properties which make them very suitable for use in pyroelectric infra-red detectors. The use of uranium as an off-valent dopant ion gives control of the electrical resistivity in the system and permits the elimination of high value gate bias resistors as discrete components. This paper discusses the mechanism by which conduction occurs in these ceramics. It is concluded from measurements of DC and AC conductivity that the uranium ions act as electron donors with conduction being via a hopping mechanism. The modelling of the AC conductivity and its use in describing device Johnson noise is discussed.