The arterial supply of the optic nerve and the retina of four fetal and ten adult sheep was studied by various injection techniques and verified by histological examination in one neonatal lamb. The optic nerve was supplied by the rostral cerebral artery, including the rostral hypophysial artery, the internal ophthalmic artery and its chiasmatic rete, and recurrent branches from the choroidoretinal arteries. The retinal arterioles were supplied by the choroidoretinal arteries, which coursed rostrally in the external sheath of the optic nerve and perforated the area cribrosa of the sclera. The dorsal choroidoretinal artery arose variably from the external ophthalmic artery and its terminal branches. It supplied the rostral portion of the optic nerve, the choroid and, except in one sheep, all retinal arterioles. The ventral choroidoretinal artery arose variably from the anastomotic branch between the external and internal ophthalmic arteries. It supplied the rostral aspect of the optic nerve and the choroid and continued in the fetal lambs as the hyaloid artery. In one specimen it also supplied the lateral retinal arterioles. There was no evidence for a central retinal artery and a continuous arterial circle of the optic nerve.