The diffusion of lidocaine to myelinated and unmyelinated axons was compared on individual afferent fibers of rabbit vagus nerve. The criterion consisted of the time required for more than 95% completion of the asymptotic increase in impulse conduction time produced by a weak, nonblocking concentration of lidocaine. Measurements on sheathed and desheathed nerves for both myelinated and unmyelinated axons detected an apparent but statistically not significant diffusional lag at the perineurial sheath, averaging four minutes in this model; there was no significant difference in the mean time for attainment of criterion in myelinated and unmyelinated axons, which averaged an additional 13 min in both types of fiber. From these observations the authors conclude that lidocaine diffused as readily through the nodal gap to the excitable membrane of the myelinated fiber as through the Schwann cell mesaxon to the unmyelinated fiber. Thus differential diffusion within a nerve seems unlikely to be a contributing factor to clinical differential block.