The reaction of ganglion cell axons in the mouse retina to optic nerve crush was studied in adult and neonatal albino mice 10-85 days after operation, using silver-stained retinal whole mounts and sagittal sections of retina and optic nerve. In both adult and neonatal animals the majority of neural cells and axons degenerated; surviving neurons had small cell bodies. Degeneration was more marked in neonatal neurons compared to adult neurons. In the adult study, two populations of axonal sprouts growing from the ends of the severed ganglion cell axons were identified. One population, representing the large majority of fibres grew for up to 20 days after operation in the myelinated retinal stump of the optic nerve and then degenerated. A smaller number of axons grew for the whole duration of the study, initially in the inner plexiform area juxtaposed to the non-myelinated optic nerve head and peri-papillary region of the retina, but later invaded the entire retina. In the neonate, no evidence of axonal regeneration was seen, although transient axonal collateral sprouting of surviving ganglion cells occurred.