Abstract: The objectives were to study the ultrastructural changes in the myelinated nerve fibers in an animal model of galactosaemia. The study was done in the Anatomy Department, Faculty of Medicine, Umm Al-Qura University, Makkah, Saudi Arabia. Twenty-four adult male albino rats were used (6 control and 12 experimental animals). Galactosaemia was induced by adding 40% D-galactose to the rats' diet for 2 months. Sciatic nerves of the control and experimental animals were removed and processed for electron microscopic study. Four months following galactosaemia, the myelinated nerve fibers showed cytoplasmic vacuoles in Schwann cells, myelin degeneration, axonal retraction and destruction of the myelinated axons. 6 months after induction, some myelinated nerve fibers showed disruption of myelin sheaths with marked shrinkage of the axons. The endoneurial edema was prominent and some regenerating nerve fibers were reported. 8 months latter: the endoneurial and intra-axonal oedema accumulated more. Schwann cells showed cytoplasmic degenerated myelin, fat vacuoles and accumulation of fine glycogen granules in the axoplasm. It could be concluded that in galactose intoxication induced degenerative changes in the myelinated nerve fibers. It showed also decrease in diameters of the myelinated nerve fibers and axons.