The firing of inhibitory interneurones, like all excitable neurones, is contingent upon how synaptic potentials are integrated and how action potentials are triggered. The electrotonic structure of a neurone, determined by its morphology and passive membrane properties, underlies the integration of synaptic signals, as well as voltage-dependent interactions between synapses (Rall, 1962). In turn, the analysis of electrotonic structure forms the framework for studying the active properties of neurones as it shapes the temporal and spatial distribution of potentials that activate non-linear conductances. The detailed examination of neuronal electrotonic structure has been performed for many types of neurones including CA1 pyramidal neurones, CA3 pyramidal neurones, neocortical pyramidal neurones, cerebellar Purkinje cells, motor neurones and CA1 interneurones (Stratford et al. 1989;Major et al. 1994;Rapp et al. 1994;Mainen et al. 1996;Carnevale et al. 1997). The general principles of cable theory, first developed by Rall (1962), are supported in all of these models (e.g. synaptic signals attenuate from the dendrite to the soma).