In 22 normal human subjects, descending lumbosacral cord potentials (DLCP) were recorded intrathecally after stimulation of the median nerve at the elbow. The onset of DLCP is very short in latency (mean 12.1 ms) with a prominent sharp early positive peak (mean latency 13.7 ms) followed by a sharp negative peak (mean 17.6 ms). The amplitude of the first part of DLCP varied between 0.6 and 6.7 microV (mean 2.3 microV). The response was recorded most easily when the tip of intrathecal electrode was posterolaterally positioned. The threshold of the response was above or around the excitation threshold of the motor nerve fibers and it could not be produced by pure skin nerve stimulation. It resisted to subtetanic peripheral shocks. Mean peripheral conduction velocity responsible for the response was about 60.8 m/s. Some late and slower deflections appeared in many cases. It was concluded that the DLCP must have originated from the descending and very fast conducting propriospinal pathways located within the anterolateral funiculus which has an oligosynaptic anatomical organization. This response seemed to be the first direct evidence of interlimb reflex action between the arm and leg in man which is important in the coordination of movements and posture.