In rat sciatic nerves, a small bundle of fibers was identified in which myelin sheaths were absent at birth, appeared within 3 days, and grew rapidly for 2 wk. During this interval, nerves were removed from littermates and were sectioned serially in the transverse plane. Alternating sets of thin and thick sections were used to prepare electron micrograph montages in which single myelinating axons could be identified and traced distally. During the formation of the first spiral turn, the mesaxon's length and configuration varied when it was studied at different levels in the same Schwann cell. The position of the mesaxon's termination shifted while its origin, at the Schwann cell surface, remained relatively constant. Along myelin internodes composed of two to six spiral turns, there were many variations in the number of lamellae and their contour. Near the mesaxon's origin, longitudinal strips of cytoplasm separated the myelin layers. Thicker sheaths were larger in circumference, more circular in transverse sections, and more uniform at different levels. Irregularities were confined to the paranodal region, and separation of lamellae by cytoplasm occurred at Schmidt-Lantermann clefts. Approximate dimensions of the bundle, its largest fibers, and their myelin sheaths were measured and calculated. The myelin membrane's transverse length and area increased exponentially with time; the growth rate increased rapidly during the formation of the first four to six spiral layers and remained relatively constant during the subsequent enlargement of the compact sheath.