The silent period evoked by the electrical stimulus of the abductor pollicis brevis and the triceps surae muscles was studied in both normal subjects and in patients affected by spinal compression, by multiple sclerosis and by familial degenerative myelopathy. In normal subjects, the duration of the silent period is shorter in the upper limbs than in the lower limbs. In patients with spinal compression and multiple sclerosis, the duration of the silent period in the lower limb is significantly greater. On the contrary, the duration in familial degenerative myelopathy is significantly longer, recording from the abductor pollicis brevis. The authors conclude that this technique is useful in the diagnosis of spinal lesions from compression and demyelination.