1. The segmental effects of spinal cord stimulation (SCS) were studied in twenty-four human subjects who had spinal cord stimulators implanted for the treatment of pain. The cathode was in the epidural space over the dorsum of the thoracic cord. 2. SCS generated action potentials in sensory, motor and mixed nerves which could be recorded with near-nerve electrodes. These action potentials could follow high frequencies of stimulation and appeared to be due to the antidromic activation of primary afferents in the dorsal columns. 3. Synaptic actions on single lumbosacral motoneurons were derived from peristimulus time histograms (PSTHs) of single motor units. SCS produced a brief short-latency period of increased firing probability (PIF) in motoneurons of all of the muscles examined, probably representing monosynaptic activation. It is argued that the facilitation arises from the antidromic activation of Ia afferents in the dorsal columns. This is the probable explanation for the muscle contractions that can be induced by SCS. 4. SCS inhibited short-latency group I homonymous facilitation and reciprocal inhibition. The mechanism appears to be presynaptic to the motoneurons and may represent collision in Ia afferents, presynaptic inhibition or homosynaptic depression. 5. It was difficult to demonstrate consistent effects of SCS on reflex pathways from cutaneous afferents to flexor motoneurons because the effects of stimulation of cutaneous nerves on these motoneurons were themselves variable. 6. It is concluded that SCS applied with epidural electrodes over the dorsal cord activates primary afferents in the dorsal columns. Antidromic activation of these afferents results in strong monosynaptic facilitation of motoneurons as well as reduction in transmission in some reflex pathways to motoneurons.