The aim of this study was to determine in monkeys if inactivation of dentate and lateral interposed deep cerebellar nuclei preferentially impairs certain movements relative to others. Constrained movements of the digits were trained with digits, hand, and elbow constrained in a cast. Simple movements were flexion of Thumb or Index. A compound movement was simultaneous flexion of Thumb+Index. An unconstrained movement consisted of a reach to, pinch of, and retrieval of a small food reward (Reach+Pinch). In two monkeys we mapped the dentate and interpositus with 66 injections of muscimol (3 microl of 5 microg/microl). Thirty-two percent of the injections resulted in increased reaction times of Thumb, Index, and Thumb+Index (mean = 24, 24, 28 + 26, respectively). Fifty percent of the injections impaired Reach+Pinch, producing target overshoot, curved reach trajectory, missed target (X and Y errors), and clumsy pinch with dropped fruit bits. Inactivation impaired each and all of Thumb, Index, Thumb+Index, and Reach+Pinch in 27%, only Reach+Pinch in 23%, and only Thumb, Index, Thumb+Index in 5% of injections. In sum, all types of movement were impaired. Thumb+Index was no more impaired than Thumb or Index alone, suggesting that the lateral cerebellar nuclei are not specifically required for combining movements of the two digits when constrained. Reach+Pinch appeared so greatly impaired and Thumb, Index, Thumb+Index so little as to be consistent with the hypothesis that a principal role of the cerebellum is to control those interactions that occur between body segments in natural unconstrained movements. However, the fact that all movements were impaired shows that the cerebellum contributes to the control of all movements.