After injury in the central nervous system of adult mammals, many of the axons that remain attached to their intact cell bodies degenerate and decrease in calibre. To understand this process better, we have investigated the relationship between axonal loss, cell loss, and the time course of changes in axonal calibre. Optic nerves (ONs) were crushed and the numbers and sizes of axons remaining close to the cell bodies (2 mm from the eye) and near the site of the lesion (6 mm from the eye) were determined for nerves examined between 1 week and 3 months after injury. Comparison with the retinal ganglion cell (RGC) counts from the same animals revealed that axonal loss was concomitant with cell body loss for at least the first 2 weeks after injury. However, there was no significant change in the calibre of the surviving neurons until 1 month after injury. Thereafter, the axonal calibre was decreased equally along the ON. No progressive somatofugal atrophy was observed. These decreases in axonal calibre occur much later than the immediate drop in neurofilament (NF) expression that also follows injury. The late effect of injury on axonal calibre suggests that NF expression is not the sole determinant of axon size of the RGC fibers in the ON. Other factors are likely additional contributing factors, such as the decreased rate of axonal transport that would help maintain the axonal neurofilament content.