We recorded motor action potentials in cats, using surface electrodes placed over the soleus muscle. The action potentials were generated by stimulating the spinal cord with electrodes in the epidural space at the level of the fifth or sixth thoracic vertebra. This also was done in humans, using the same methods of stimulating and recording, but the intensity of the stimulus was adjusted to produce little or no twitch of the paraspinal muscles. In the animal experiment, the motor action potential was abolished after transection of the pyramidal tract and was progressively attenuated with effective doses of a curare-like agent. We also tested the effect of distraction, using the same technique as is used in Harrington instrumentation, and found that the amount of distraction that caused reduction of the amplitude of the motor action potential of more than 50 per cent, when sustained for longer than seven minutes, caused permanent paraplegia in two cats. The evaluation of spinal evoked potentials that were obtained from epidural electrodes placed caudad to the level of distraction, and of motor action potentials that were recorded over the soleus muscle, following the same stimulus, showed a similar pattern of reduction after distraction in five of seven cats. The other two cats had irreversible reduction of motor action potential associated with unchanged spinal evoked potential, and both cats became paraplegic.(ABSTRACT TRUNCATED AT 250 WORDS)