Axon number in the splenium was examined at 15, 25 and 60 days of age in male and female rats. The splenium (posterior fifth) of the corpus callosum was found to contain the axons from the visual cortex at all three ages and was extensively sampled with electron microscopy. Overall, there was a 15% decrease in the total number of axons between postnatal day 15 and day 60 in both sexes. The observed decrease in axon number between day 15 and 25 in both males and females is consistent with Elberger's (A.J. Elberger, Transitory corpus callosum axons projecting throughout developing rat visual cortex revealed by DiI, Cereb. Cortex 4 (1994) 279-299) data which suggest that the pattern of visual callosal projections in the rat visual cortex is not restricted to the adult form until the fourth postnatal week. There was a further decrease in axon number between day 25 and day 60 in females only such that by 60 days of age, the total number of axons was equivalent between the sexes. Thus in the rat splenium, males appear to attain the adult number of axons earlier than females. These results also indicate that there is a sex difference in the timing of axon withdrawal in the rat splenium, with axon withdrawal continuing in females after it has ceased in males.