The properties of neurones of the guinea-pig deep cerebellar nuclei in a slice preparation were investigated by intracellular recording. The recorded population of cells did not differ morphologically from nuclear cells in vivo as judged from neurones stained with Lucifer Yellow. Fifty-two out of sixty cells were spontaneously active with a regular firing pattern and a mean frequency of 26 +/- 14 (mean +/- S.D.) impulses/s. The action potentials lasted 0.41 +/- 0.07 ms (n = 60) with an amplitude of 58 +/- 8 mV. Input resistance was 44 +/- 10 M omega and the time constant of the membrane 13 +/- 3 ms. When stimulated with intracellularly injected depolarizing current pulses the cells responded with trains of action potentials. Near the threshold for the spike the stimulation produced firing of constant frequency and from more hyperpolarized levels an initial acceleration sometimes followed by a deceleration was seen. At levels less than 15 mV from the spike threshold there was a rebound train of spikes as a response to a hyperpolarizing current injection. At more hyperpolarized levels there was only a small depolarizing potential after the hyperpolarizing stimulation. Three types of subthreshold potentials were recorded. Spikelets rose from base line as 3-10 mV depolarizing wavelets with a duration between 5 and 10 ms. They served as trigger potentials for the action potential. Plateau potentials were slow depolarizing potentials often reaching the spike threshold and thus generating long trains of action potentials. After-hyperpolarizations followed each spike with a time course dependent on the previous activity of the cell. Plots of the firing frequency versus injected current were linear at the first and second interspike interval, after 50 ms of activity and at steady state. Plots of the voltage versus injected current were upward concave demonstrating anomalous rectification of the cell membrane. It is concluded that neurones in the deep cerebellar nuclei in vitro are spontaneously active because of the electroresponsive properties of their membranes. The physiological importance may be that the cerebellar output from these cells can be rapidly and efficiently modulated by synaptic potentials generated by Purkinje cells and mossy and climbing fibres.