The relationship between peripheral conduction velocity and the configuration of the soma action potential was examined in rat lumbar dorsal root ganglion neurones during the first two weeks of postnatal development. The results show that the soma membrane properties of A cells change as their conduction velocity increases during myelination, but that C cells retain the same soma membrane properties they express at birth, while their conduction velocity increases only slightly. It is suggested that the relationship between primary afferent axons and their associated Schwann cells may play a role in the differentiation of soma membrane properties of sensory neurones.