Schwann cells are frequently present in retinae of a partially inbred strain of rats, the BW rats. In affected animals, most if not all axons in the nerve fibre layer, immediately adjacent to the optic disc, are ensheathed by Schwann cells. This can occur in the form of a distinct myelin sheath or a simple envelopment of the axons by Schwann cells. Some axons lose their Schwann cell sheath within a few hundred micrometers of the optic discs while others retain the sheath out to the mid-retina. Although the pathophysiological basis of the defect is unknown, it appears to be related to a genetic defect that has widespread C.N.S. expression.