The ventral funiculi of normal and X-irradiated 13-day-old rats were studied by electron microscopy. In both tissues, oligodendrocytes form myelin sheaths around multiple axons, with a single oligodendrocyte associated with several axons of different sizes. Despite their origin from the same glial cell, the myelin sheaths are thicker for larger axons. Polyribosomes and rough endoplasmic reticulum are observed in distal oligodendrocyte processes, in proximity to the forming myelin sheaths. These results indicate that myelin sheath thickness is matched to axon size via local mechanisms, and suggest a role of polyribosomes and/or rough endoplasmic reticulum in myelin formation.