In order to clarify the motor functions of the substantia nigra (SN), we studied the activity of single neurons in both the pars compacta (SNpc) and the pars reticulata (SNpr) of behaving primates during performance of a visuomotor arm-tracking task. We also studied the relations of neuronal activity to active movements and passive manipulations of the limbs and other body parts outside the behavioral paradigm.On the basis of spontaneous discharge rates, most cells within the SN fell into two categories: (1) low discharge rate (LDR) cells (n = 53, mean rate 2. 1 impulses/set) and (2) high discharge rate (HDR) cells (n = 129, mean rate 60.4 impulses/set). HDR neurons had firing rates and discharge patterns similar to neurons in the inner segment of the globus pallidus. Most (83%) LDR neurons were located within the SNpc, and all HDR neurons were located in the SNpr. None of the LDR neurons in SNpc exhibited detectable phasic changes in discharge in relation to active movements or passive manipulations. Only a small number of SNpc cells showed modulation in the behavioral paradigm. These findings suggest that the nigrostriatal dopamine system, rather than conveying highly specific information about movement to the striatum, may exert a more tonic modulatory effect upon the striatum. However, phasic release of dopamine in the striatum may be affected by presynaptic mechanisms.Twenty-five percent of cells in the SNpr were related to licking and chewing movements. Cells specifically related to limb movements were rare. Some of these arm movement-related neurons showed a relation to movement parameters. A small number of SNpr cells (3%) were related to eye movements or exhibited responses to gross visual stimulation. Neurons related to licking and chewing were located primarily in the centrolateral portions of the nucleus. These findings suggest that the lateral portion of the SNpr may play a role in the control of orofacial movements.