Due to the motor evoked potentials recorded in limb muscles after transcranial and spinal magnetic stimulation, conduction in the central motor pathways can now be evaluated safely and painlessly in man. The central motor conduction time obtained includes the time required for transmission, along the fast pyramidal fibres, from the cortex to the spinal motoneurons, the synaptic transmission to motoneurons and the conduction on a short segment of the motor nerve root. Lengthening of this time almost always reflects dysfunction of the central motor pathways. The abnormalities observed are not specific of any particular cause, and they must be interpreted in relation to the context. The usefulness of this new electrophysiological technique is being tested by radiological and anatomico-clinical correlations in various diseases of the central nervous system (e.g. disseminated sclerosis, cerebral infarction, spinal cord injury) and the locomotor apparatus (e.g. cervical myelopathy, radiculopathy).