Myelinated sensory afferents on entering the spinal cord form a junction in the dorsal columns and send a branch toward the head and another caudally. Recent experiments have shown that orthodromic impulses are reliably transmitted over the rostral branch but may suffer a transmission block in the caudal branch. This paper investigates a possible reason for the difference between rostral and caudal branches of the same axon by measuring the conduction velocities in each. Axons of either the ascending or descending branches of afferents in the L1 dorsal root of rat spinal cord were stimulated in the dorsal columns at various distances from the dorsal root. The resulting compound action potential or single unit spikes were recorded on the L1 dorsal root. The conduction velocity was found to be twice as fast in the rostral branch as in the caudal branch of the same axons.