Myelination has been studied in th corpus callosum of the mouse brain between birth and 240 days-of-age. Myelin sheaths were first seen at 11 days. The most rapid phase of myelination occurred between 14 and 45 days when 13.5% of axons were myelinated, but myelination continued at a reduced rate up to 240 days when 28% of axons were myelinated. The mean diameter of unmyelinated axons was more or less constant throughout the study with an overall mean diameter of 0.25 +/- 0.01 micron. Similarly myelinated axon diameter showed little variation with age with a mean diameter of 0.46 +/- 0.01 micron. This suggests that in the corpus callosum axons do not increase in size until they begin to myelinate.