Several factors that influence the inhibition of primate spinothalamic tract (STT) cells produced by repetitive peripheral conditioning stimulation have been studied. Identified STT cells were recorded from the lumbosacral spinal cord in intact, anesthetized monkeys. In addition, presumed STT cells were recorded from unanesthetized, decerebrate or decerebrate, spinalized monkeys; these cells were identified by antidromic activation from the contralateral ventral lateral funiculus of the upper cervical spinal cord. Activity of the STT cells was evoked by electrically stimulating the sural nerve with pulses having an intensity strong enough to activate C fibers. The C fiber evoked STT cell activity was compared before, during and after repetitive conditioning stimuli applied to the tibial nerve for 5 min. By applying repetitive strengths of conditioning stimuli, it was found that the A delta fiber group is the most important for producing inhibition of STT cells, although significant additional effects were also produced by the A alpha beta and C fiber groups. Conditioning stimuli with fixed intensity at different frequencies showed that the higher the frequency the more powerful the inhibition within the range we tested (0.5-20 Hz). The inhibition produced by peripheral nerve stimulation was segmentally organized, so the most effective nerve in producing inhibition amongst those tested was the ipsilateral tibial nerve. The contralateral sciatic nerve, the ipsilateral median nerve and the contralateral median nerve were less effective in that order. The results of the present experiments suggest that the most effective way to produce analgesia by peripheral nerve stimulation would be by high frequency stimulation of a nerve innervating the area from which pain originates with an intensity at least strong enough to activate A delta fibers.