In decerebrate, unanesthetized cats, the activity of a Purkinje cell and a paired neuron in the interposed nuclei (subsequently referred to as a Purkinje cell-nuclear cell pair) were simultaneously recorded to compare their spontaneous discharge characteristics and responses to a peripheral input. Microstimulation techniques were used to determine if the two neurons were in related regions of the cerebellar cortex and interposed nuclei. First, the electrode used to record the interposed nuclear neuron was used to antidromically activate the Purkinje cell. Second, inhibitory responses with the appropriate time course were evoked in the interposed neurons with the Purkinje cell recording electrode. Both the spontaneous discharge of each pair as well as the cells' responses to a mechanically generated forepaw displacement were evaluated. A total of 35 Purkinje cell-nuclear cell pairs satisfying the microstimulation criteria were studied. During spontaneous activity there was no consistent relationship between the Purkinje cell simple spike activity and the nuclear neuron activity based on cross-correlation techniques. Auto-correlograms of both cells exhibited positive correlation for a brief time period. The variability of the interspike interval of the Purkinje cell discharge was greater than that for nuclear neurons, but there was no difference in the mean interspike interval for the pairs. The simple spike activity of Purkinje cells in response to sinusoidal displacements of the forepaw was weakly modulated. The modulation was limited to a small frequency range (2-7 Hz). In contrast the nuclear neurons exhibited a greater depth of modulation than Purkinje cells and responded to a wider range of frequencies (2-15 Hz). For many pairs of cells the relationship between the Purkinje cell and nuclear neuron discharge was not reciprocal. The responses of Purkinje cell-nuclear cell pairs to forepaw displacement were more often reciprocal to square wave than to sinusoidal stimuli. Using a technique which estimated the reciprocity of Purkinje cell and nuclear cell responses, 49% of the responses were reciprocal while 51% were not. These findings suggest that a nuclear neuron's discharge characteristics are not dominated by inputs from a single Purkinje cell and the firing relationship between a Purkinje cell and a related nuclear cell need not be reciprocal.