Three patients with well-mapped, circumscribed ablations of the medial part of the frontal lobe are reported. A specific clinical evolution in 3 stages was observed: (1) post-operatively there was global akinesia, more prominent contralaterally, with an arrest of speech; (2) recovery occurred suddenly but even then there was severe reduction of spontaneous motor activity contralaterally, an emotional type facial palsy and a reduction of spontaneous speech; (3) long after the operation the only sequela was disturbance of the alternating movements of the hands. The anatomical basis of these disorders appears to be a lesion of the supplementary motor area (SMA). Three main points have been discussed, namely: (1) the nature of the motor disorder which may be a disturbance of the function of the SMA in initiating and sustaining spontaneous and automatic motor activity; (2) the nature of the arrest of speech which appears not to be aphasic; the studies reported in the present paper do not support a left cerebral dominance for the SMA as suggested by previous studies; (3) the method of clinical examination appears to be important in distinguishing between spontaneous and voluntary motor and speech performance. Differences in method may account for differences between our findings and those reported in previous studies.