In the majority of mammals axons of retinal ganglion cells are not normally myelinated intraretinally. To test the hypothesis that the lamina cribrosa normally prevents myelin-forming cells from entering the retina we have examined the axons of retinal ganglion cells in conditions where there is no lamina cribrosa. Following transplantation of fetal retinae to the midbrain of newborn rats we have shown that ganglion cell axons within the transplants subsequently become myelinated, providing further evidence that the intraretinal segment of a ganglion cell axon is not refractory to myelination if myelin-forming cells are allowed access. Thus, our results support the hypothesis that the lamina cribrosa normally prevents oligodendrocytes or their precursors from gaining access to the retina. A number of factors may be involved in restricting the migration and differentiation of myelin-forming cells but it is apparent that there is a correlation between the absence or paucity of myelination and the presence of locally increased permeability of the blood-brain barrier. We suggest that proteins derived from plasma may influence oligodendrocyte precursor migration and/or differentiation at these sites.