In bovine ocular tissues, cornea and retina-including choroid provide the intracellular proline synthetic pathway from ornithine, but not from glutamate. In C3H retinal degeneration mice, P5C reductase activity in the retina and choroid was decreased to about one-third that of CRJ control mice in the specific activity after the tenth day. The activity of an incorporation of 14C-ornithine into proline in the retina and choroid was also decreased to the same extent after the weaning period, in the retinal degeneration mice. Postnatal changes of P5C reductase and ornithine oxoacid aminotransferase activities in the livers of C3H mice showed a pattern that reflected a delayed weaning. Thus, possibly due to the blood-retinal barrier, "an abiotrophy of some membrane formation" resulting from a proline deficiency at the locus may be attributable as an important factor for the development of retinal degeneration.