Anodal shocks of 400 to 700 V from a low-output impedance stimulator applied percutaneously over the motor cortex evoke muscle action potentials in partially voluntarily activated contralateral muscles. Cathodal shocks from the same device applied to the cervical spinal cord produce maximal ipsilateral muscle action potentials in a relaxed limb. This technique was used to study the central motor pathway in 15 healthy subjects and 8 patients with clinically definite multiple sclerosis. As stimuli were applied in the axilla, over the C7 vertebral level, and over the arm area of the motor cortex, recordings were made of muscle action potentials of forearm flexor muscles. In controls, cord-to-axilla conduction time was 4.1 +/- 0.61 ms, and cortex-to-cord time was 4.4 +/- 0.75 ms. In patients, cord-to-axilla conduction times were normal, while central conduction times were either markedly prolonged (6.4 to 31 ms) or absent. This technique is a potentially powerful tool for the investigation of central motor pathways in healthy subjects and patients with neurological disease.