The relation between fiber size and safety of conduction was testedin vitroon individual afferent axons of rabbit vagus nerve by lowering the external sodium ion concentration and noting the effect on threshold excitability and conduction velocity. Conduction safety of myelinated axons was found to be independent of fiber size and slightly less than among unmyelinated axons. The results are consistent with previous data from the same model, where blocking concentration and diffusion time of lidocaine to the excitable membrane of individual axons also were independent of myelinated axonal size. The evidence from these single-unit studies implies that the differential blocks of functional modalities observed with spinal and epidural anesthesia probably do not arise from fiber size-related differences in susceptibility to block: possible alternatives are mentioned briefly.