The H reflex plays an important role in electrodiagnosis, but it does not give specific information about any particular segment of its long conduction pathway. Direct stimulation at the S1 spinal nerve might provide more complete information by dividing the H reflex pathway into its peripheral and spinal conduction portions. Patients with electromyographic evidence of S1 radiculopathy (n = 77) and normal control subjects (n = 56) were evaluated. Standard H reflex conduction studies were followed by monopolar needle stimulation of the first sacral spinal nerve. The ratio of the spinal nerve latency to the H reflex latency (S1 ratio) was calculated. The abnormal S1 ratio seen in 77% of the subjects suggested relative slowing within the spinal segment of the nerve. The remainder of the subjects with S1 radiculopathy had slowed conduction also affecting the peripheral segment that could have been caused by Wallerian degeneration. The S1 ratio can provide evidence that H reflex conduction slowing is the result of injury involving the intraspinal nerves. This technique should be especially useful in cases of acute lumbosacral injury when needle electromyographic study is often nondiagnostic.