Neurophysiological methods which allow to identify objectively lesions of the efferent cortico-spinal pathways in humans are with the exception of transcutaneous electrical stimulation of the motor cortex nor available. Long latency EMG responses from leg muscles are mediated by a "transcortical loop" and offer the possibility to detect in combination with sensory evoked potentials lesions both of the afferent proprioceptive and the efferent motor pathways. Subjects stood on a platform which was rotated toe-up (50 degrees/s, 4 degrees) around the axis of the ankle joint. This lead to two EMG responses of short- and medium latency in the stretched triceps surae muscle, and an EMG response of long latency (LL) from the antagonistic anterior tibial muscle. We investigated 135 patients with a spinal or cerebral lesion, detected by neuroradiological methods. We found a significant delay of LL in patients with spinal lesions and in patients with cerebral lesions on the affected side. A significant delay of LL could also be observed in patients with pure motor symptoms and normal latency of the cerebral potential (P 40) alone in the group of patients with spinal lesions revealed in about 45% a pathological result. The rate of pathological findings was about 65% when both methods (SEP and LL) were combined. The results indicate that the recording of LL is helpful to detect lesions of the efferent central pathways. In a follow-up study we investigated patients who underwent successful surgical treatment. We found a close correlation between the decrease in latencies of LL and the improvement of clinical signs.