Changes with age were investigated in the anterior and posterior limbs of the anterior commissure of the brains of s-mo-old and 18-mo-old mice. In the 18-mo-old animals the total number of glia was reduced by 40% in both limbs. All types of glia were reduced in numbers, the astrocytes being most affected. Ultrastructural changes were also most obvious in astrocytes. There was no significant change in the number of axons, percentage myelination, axonal diameter (myelinated or unmyelinated) nor in the mean number of myelin lamellae in either limb in the 18-mo-old animals. The percentage of redundant myelin sheaths, however, increased significantly in 18-mo-old animals.