1. Recordings were made of the natural discharges of neurons in the supplementary motor area (SMA) of conscious monkeys trained to perform stereotyped motor task, pulling a horizontal lever, with either hand. 2. Of the total population of cells, 80% showed modulation of their activity during particular movements of either limb. Many cells had a similar pattern of modulation regardless of whether the contralateral or ipsilateral hand was used. Of the remaining 20%, some cells were related to leg or body movements or to visual experience. 3. Cells whose activity was related to movements of distal joints were found in approximately equal numbers to those whose discharges occurred with proximal movements. 4. Only 5% of cells tested sent their axons into the pyramidal tract, and only 14% of units investigated showed responses to passive manipulation of the limbs. The effective afferent input usually was of a rather complex kind. 5. The findings suggest that the discharges of a large number of neurons in SMA are changing during particular movements of either arm, and that only a small number of cells receive peripheral afferent sensory input. These results contrast with those obtained in the primary motor area and suggest a different role for SMA in the control of movement.