We describe the relations between the direction, amplitude, and velocity of step-tracking arm movements and the frequency of single cell discharge in the external (GPe) and internal (GPi) segments of the globus pallidus and the subthalamic nucleus (STN) of the behaving monkey. Statistically significant relations to the direction, amplitude, and peak velocity of the movement were found in all structures studied predominantly during the movement but also during the reaction time. For movements in a particular direction, the discharge rate was frequently a linear function of the movement amplitude and/or peak velocity. The slopes of this relation differed for different cells and comprised both positive and negative values. STN differed from both GPe and GPi in that (a) a larger proportion of neurons in STN showed significant relations to the direction of movement and (b) the onset times of changes in neural activity related to movement occurred earlier in STN than in GPe or GPi. The results of these studies suggest that cells in GPe, GPi, and STN may be involved in the control of movement parameters. Loss of the basal ganglia output related to the amplitude or velocity of movement might account for the impairments of step movements observed in Parkinsonian patients. On the other hand, deranged or excessive output related to amplitude or velocity control might result in the excesses of movement observed in other disorders, such as chorea and hemiballismus. These studies also provide direct evidence that the STN exerts a specific influence on basal ganglia output related to the control of movement parameters.