Unilateral degenerative atrophy of the optic nerve (ON) occurred in 6 of 80 male and 4 of 80 female Slc: Wistar rats. Two of these cases completely lost the intracranial portion of the unilateral ON and the remainder had the small ON. The optic disc and ON were located histologically in the posterior pole of the eyeball of 2 rats with no intracranial ON. ON lesions in all cases were characterized by a reduced number of axons with a small number of myelinated axons and marked astrogliosis. There were also swelling, fragmentation and spheroid formation of axons, as well as thickening of the connective tissue sheaths and vessel walls in the ON. One side of the optic chiasma and optic tract contralateral to the affected ON reduced in volume, became degenerated and were accompanied by gliosis. Focal of diffuse degeneration of the retina was observed in the eyeballs with affected ON. Retinal ganglion cells decreased in the number showing chromatolysis. These retinae became thin and developed degeneration of both inner and outer portions with sclerotic changes in the retinal vessels. The ophthalmic and ciliary arteries in the eyeballs with affected ON often developed broliferative or occlusive. endoarteritis, suggesting that retinal lesions may have resulted not only from axonal degeneration in the ON but also from ischemia. Histologic lesions suggestive of transneuronal degeneration were found in the contralateral lateral geniculate body and rostral colliculus. Based on the data presented, it was presumed that a primary lesion may have been induceds in the ON by a circulatory disturbance and followed by retrograde and anterograde degeneration in the visual pathways.