The development of Schmidt-Lanterman incisures was investigated in the rat sural nerve during an active phase of postnatal myelination (5-21 days post partum). Two distinct populations of incisures were recognised and the following nomenclature for their developmental stages is proposed. Primary incisures which appear ab initio in myelination and always extend across the whole radial thickness of the myelin sheath but initially around only part of its circumference. Consequently they appear in transverse section as sectoral incisures (occupying less than half the circumference) and in longitudinal section as asymmetric incisures (involving one side only of the myelin sheath). Secondary incisures appear later, in regions of a compact myelin sheath, initially traversing only part of its radial thickness but commonly occupying its whole circumference. Thus they usually appear in transverse section as circumferential incisures and in longitudinal section as symmetric incisures (involving both sides of the myelin sheath). Less commonly secondary incisures may form in a sector of the myelin sheath but still in regions of compact myelin and thus appear asymmetric in longitudinal section and sectoral in transverse section. Secondary incisures appear mainly adaxonally in the earlier stages examined and mainly abaxonally in the later stages. The maturation of primary and secondary incisures into the radially and circumferentially complete incisure characteristic of the mature myelinated nerve fibre is described. The above mechanisms of incisural formation are contrasted with mechanisms previously suggested to occur during normal development and remyelination and related to the plasticity and ultrastructure of the myelin sheath.