Unilateral and bilateral dysplasias of the optic nerve (ON) were observed in 20/114 male and 14/110 female Sprague-Dawley rats at 12 weeks of age. Grossly, the intracranial segment of the affected ON had nodular thickening, bifurcation, and curvature. Nodular thickenings were seen in 20 males and 11 females. One female had a bifurcated ON. Curvature was observed in the left ONs of two females. Of 34 ON dysplasias, 12 ONs tapered off into a thin filament at the portion anterior to the dysplastic lesions. The intraorbital segments of the ONs in 33 rats were also reduced in size and were hardly recognizable in the meningeal sheath in 10 rats. Both eyeballs appeared normal in all the animals examined. Histologically, nerve fibers in intracranial and intraorbital segments of the ONs that appeared as slender filaments were markedly reduced in number. Nerve fibers in nodular thickenings were intertwined in haphazard fashion, forming scrollworklike structures. The meningeal sheaths in intracranial segments of the ONs in 15 rats and in intraorbital segments in eight rats were partially missing. The naked portion of the ON protruded into the meningeal spaces or gaps. The data indicate that developmental failures in the ON may have been induced due to insufficient blood supply through the meningeal covering or herniation of growing nerve fibers into the defective meninges. However, etiology and pathogenesis of this condition remain unclear.