Measurements of cerebral blood flow in man revealed that complex voluntary movements are associated with a blood flow increase in the supplementary motor area of the brain. This increase is additional to and similar in magnitude to the Rolandic sensorimotor area activation that occurs during all kinds of movement. When subjects counted silently there was no activation of any focal cortical area in the brain; when they counted aloud there was a marked increase in activity in the supplementary motor area. These results are consistent with the hypothesis that the supplementary motor area plays a major role in the initiation and control of at least some kinds of voluntary movement in man and is, therefore, a motor center of a higher order than the primary Rolandic areas.