Recent anatomical evidence suggests that descending projections from the lateral vestibular nucleus (LVe) and gigantocellular reticular nucleus (Gi) innervate areas of the lumbar spinal cord near the spinal nucleus of the bulbocavernosus (SNB). To confirm this finding electrophysiologically, we recorded and mapped averaged field potentials within the lumbar spinal cord of male rats in response to electrical stimulation of the LVe or Gi and compared these with the location of averaged field potentials evoked at the same levels by stimulation of SNB axons in the bulbocavernosus (BC) nerve. Stimulation of the LVe or the Gi produced negative field potentials that were largest at sites 200-450 microns dorsolateral to SNB somata. In an attempt to verify that this region innervates SNB motoneurons, the BC motor nerve volley was recorded in response to microstimulation at various depths within the spinal cord. Stimulation of sites dorsolateral and lateral to the SNB somata elicited volleys in the BC nerve that had two components. The onset latency of the earlier component was similar to the antidromic latency of SNB motoneurons to BC nerve stimulation, and the threshold for eliciting this component was lowest at sites in the electrode track near SNB somata. Thus, the earlier component may be evoked by direct stimulation of the SNB motoneurons. The threshold for evoking the later component was lowest at the sites 230-380 microns dorsolateral to SNB somata, suggesting that this component involves activation of other neurons. These results indicate that the LVe and Gi may modulate the activity of SNB motoneurons through interneurons located in a region several hundred microns away from SNB somata.