We studied changes in basal ganglia neuronal activity associated with reaching movements of the arm in two monkeys. Data were obtained from 427 single neuronal units in putamen, 199 in caudate nucleus, and 216 in globus pallidus with multiwire electrodes allowing simultaneous recordings from multiple neurons. In all structures, changes in activity related to movement occurred most often after the onset of EMG: 43% of tested neurons in the putamen, 32% in the caudate nucleus, and 38% in the globus pallidus. Less frequently, changes began before EMG activation: 20% of neurons in the putamen, 19% in caudate nucleus, and 17% in globus pallidus. In general, these changes in neuronal activity lasted longer than EMG activity associated with reaching. The proportions of neurons activated were significantly larger in the putamen than the caudate nucleus. In the pallidum, the proportions were not statistically different from either the putamen or caudate nucleus, and no significant difference was found between the internal and external pallidal segments. Significant selectivity for movements to different targets was observed in 36% of neurons in the putamen, 28% in the caudate nucleus and 9% in the globus pallidus. The lower proportion in the globus pallidus compared to the striatum was significant (P < 0.002). Clusters of activated neurons were found in the striatum, however, the timing of changes was often different for individual neurons in these clusters. A cross-correlation analysis of the activity of neurons in the clusters revealed no evidence of common inputs, suggesting that striatal neurons in close proximity with neurons showing similar changes in activity are driven by different populations of neurons. In the putamen, the anatomical locations of neurons with changes in activity related to movement execution were on average significantly more posterior and lateral than neurons with changes related to the preparation of movement described earlier. These findings support the view that the putamen and the caudate nucleus contain distinct functional areas. The present studies show that most anatomical regions in both the striatum and pallidum participate in the control of executing reaching movements.