The ultrastructure of non-myelinated ganglion cell axolemma in the retinal nerve fibre layer from developing and adult rats was examined by freeze-fracture electron microscopy. The axolemma of fibres from neonatal (2-8 days) rats had a moderate density (510-556/micrometers 2) of intramembranous particles (IMPs) on the P-fracture face, while there was a low density (101-146/micrometers 2) of particles on the E-fracture face. Particle density on the P-face increased with development, such that by 28 days the density of IMPs was 1281/micrometers 2. Adult fibres had a high (1741/micrometers 2) density of particles on the P-face. On the E-face, the density of IMPs did not change substantially throughout development, and remained less than 225/micrometers 2 at all ages. Mean particle diameters were compiled for P- and E-fracture faces at the various developmental ages and were greatest in fibres from adult animals. P-face particle density of non-myelinated axons in the retinal nerve fibre layer changed at approximately the time that myelination occurred in distal (optic nerve) segments of these axons. The alteration in membrane structure during development of non-myelinated axons in the retinal nerve fibre layer suggests that conduction properties may also change with development.