Differential block of peripheral nerve fibers was attempted in vitro by a new approach based on inhibiting the membrane pump with ouabain. Sequential concentration dependent extinction of the components of the compound action potential was obtained: C extinguished first, A delta next, A beta last. The sequence conforms to expectations based on axonal size. Because pain is mediated by C and A delta fibers, and block of these groups by ouabain was not reversed readily, further investigation of the practicability of the new approach seems warranted.