We report several studies on the effects of removing the medial premotor cortex (supplementary motor area) in monkeys. The removal of this area alone does not cause either paralysis or akinesia. However, the animals were poor at performing a simple learned task in which they had to carry out an arbitrary action: they were taught to raise their arm in order to obtain food in a foodwell below. They were impaired whether they worked in the light or the dark. They were impaired when they had to perform the movements at their own pace, but much less impaired when a tone paced performance. Monkeys with lesions in the anterior cingulate cortex were as impaired as monkeys with medial premotor lesions at performing this task at their own pace. However, monkeys with lateral premotor lesions were less impaired. We conclude that the medial premotor areas play a crucial role in the performance of learned movements when there is no external stimulus to prompt performance.