The responses of cerebellar nuclear cells to extracellular stimulation in a slice preparation were studied and the ionic basis of their electroresponsiveness was investigated with blockers of membrane conductances and with ion substitutions in the extracellular medium. The cells could be activated antidromically from the cerebellar cortex and the white matter surrounding the nuclei. The dominating response to orthodromic stimulation was an inhibitory synaptic potential presumably produced by activation of Purkinje cell fibres. The action potentials and the subthreshold spikelets were shown to be Na+ dependent and are presumably generated by a voltage-dependent inactivating Na+ conductance. Plateau potentials with a low threshold were also Na+ dependent, but these long-lasting potentials are probably produced by activation of a voltage-dependent non-inactivating Na+ conductance. Plateau potentials with a high threshold and high-threshold spikelets were Ca2+ dependent and seem to be generated by non-inactivating and possibly inactivating Ca2+ conductances. The spike after-hyperpolarizations had an early voltage-dependent K+ component and a late Ca2+-dependent K+ component. They are therefore produced by voltage-sensitive and Ca2+-dependent K+ conductances. By analogy with the distribution of conductances in Purkinje cells it is proposed that the Na+ conductances are mainly located in the somatic and axonal membrane and that the Ca2+ conductances are located in the dendrites. The functional implications of the complex electroresponsive properties of cerebellar nuclear cells are discussed.