The slow positive (P2) wave of the evoked electrospinogram was recorded from the dorsal epidural space in man. The waveform characteristics of the P2 wave were similar to those of the dorsal cord positive wave (P wave), which is believed to be caused by the primary afferent depolarization (PAD) and to be related to presynaptic inhibitory action in animals. The "second" component of the P2 wave appeared during excitement or following strong stimulation and disappeared after thiamylal administration and during natural slow-wave sleep. The second component, also demonstrated in the P wave of rabbits during ketamine anesthesia, was abolished by spinal transection. Therefore, these second components in man and rabbits may originate from a feedback loop via supraspinal structures. Thus, supraspinal influences might play an important role in the regulation of presynaptic inhibition in the spinal cord of man during wakefulness and anesthesia.