There is evidence that peripheral nerve fibres, both myelinated and unmyelinated, have different membrane properties correlated with their size. Smaller fibres have a lower conduction velocity, and longer action potential time course, than would be expected if their membrane properties were identical with those of larger size. There are several possible explanantions of this result; it is suggested that at least part of the difference is due to a systematic variation in the size of the conductances (per unit area of membrane) responsible for generating the action potential.