Single-unit recordings were obtained from 404 neurons in the dentate and interposed nuclei in two monkeys trained to perform simple movements of the elbow in response to three different sensory cues: a light, a tone, and a small, brief perturbation of the trained forearm. Both flexion and extension movements were investigated. Those dentate neurons that showed a clear modulation before the onset of movement (149 of 318 cells recorded) were classified as stimulus related or movement related on the basis of an analysis of the timing of the initial change in discharge. Seventy-one percent of these dentate neurons (106/149) were classified as stimulus related, and 21% (31/149) were classified as movement related. Within the stimulus-related group 87% responded selectively to only one or two of the sensory cues (selective stimulus-related neurons), most often the teleceptive cues, whereas the remaining 13% responded nonselectively to all three cues (nonselective stimulus-related neurons). Interposed neurons, in contrast, showed principally movement-related discharge, and this represented the initial change in discharge in 89% of the neurons. Eleven percent of the interposed cells showed a selective response to the somesthetic cue. The discharge of 28 out of 91 dentate neurons tested with both flexion and extension movements varied with the direction of movement. Few dentate neurons (9%) were found to display any direction sensitivity when considering the discharge preceding the onset of movement, and none of these showed a reciprocal pattern. The discharge of a greater proportion of neurons (24%) was direction sensitive during movement and was occasionally reciprocal. In the same monkeys, however, 78% of the neurons in the neighboring interposed nucleus were direction sensitive, and one-quarter of these displayed reciprocal patterns of discharge. Thus, the discharge of dentate neurons, occurring well in advance of a conditioned movement, cannot specify direction in this simple reaction-time (RT) task. The sensory responses of selective stimulus-related dentate cells ended near the onset of movement but were time locked to the stimulus and not to the movement. When a neuron was responsive to two of the cues the response did not vary with the modality of the stimulus apart from changes in the latency. The initial sensory response was usually followed by later "secondary" changes in discharge that were temporally related to the movement.(ABSTRACT TRUNCATED AT 250 WORDS)