The H reflex in the S1 spinal nerve has been used in electrodiagnosis of S1 radiculopathy for several years. Direct stimulation of the S1 spinal nerve has provided more complete information about the H reflex pathway by dividing it into its peripheral and central (or spinal) conduction portions. A previous study compared spinal nerve latency with the H reflex latency, demonstrating an abnormal S1 ratio in subjects with S1 radiculopathy, thereby suggesting slowing within the spinal segment of the nerve. No study, however, has established a normal value for the central (spinal) portion of the H reflex. We electrodiagnostically tested 20 subjects with normal clinical neurologic and musculoskeletal examinations to define a normal sample of the central loop of the H reflex in the S1 spinal nerve. The peak latencies of the M and H reflex responses were measured after a single stimulus to the S1 spinal nerve. The data obtained established 7 +/- 0.3 ms as the normal value for the interpotential latency difference (central loop) of the H reflex in the S1 spinal nerve in healthy subjects. Six patients with clinical and electromyographic evidence of S1 radiculopathy all had central loop latencies of > 8 ms. The normal value of the central loop of the H reflex suggested in this pilot investigation may, therefore, be used to allow for earlier and more accurate diagnosis of an acute S1 radiculopathy.