The relation between internodal length (L) and fibre diameter (D) was examined light microscopically in teased specimens from normal developing rat sciatic nerves, and from rat sciatic nerves which had regenerated following crush lesions at various postnatal ages. In newborn rat pups virtually all sciatic nerve axons are unmyelinated and myelination is an essentially postnatal event. Between 2 weeks and 6 months maximal L increases from 500 microns to 1400 microns and maximal D increases from 6-8 microns to 16 microns. The increase in L matches the length growth of the hindlimb. Signs of myelin sheath remodelling are absent during normal development. Examination of regenerated nerves showed that the lengths of the internodes along large-medium-sized axons, are strongly dependent on crush age. In neonatally crushed nerves, the slope is close to normal. With increasing crush age the inclination of the regression line gradually decreases. Signs of myelin sheath remodelling are not seen in regenerated nerves crushed at birth or 1-2 weeks after birth. However, such signs are present in regenerated nerves crushed 3 weeks or more after birth. These observations support the view that myelin sheath remodelling in regenerated rat sciatic nerves is directly related to a deficient length growth following myelination.