The number of neurons and glia in the mouse abducens nerve nucleus was estimated at 6, 25, 28 and 31 months of age. There was no significant variation in either the number of neurons (mean 118) or glia (mean 168) or in the glia to neuron ratio (mean 1.42) with age. Mean neuronal and glial nuclear diameters also remained constant and there was no obvious loss of Nissl substance and only a minimal accumulation of lipofuscin in neuronal perikarya with age. The apparent lack of age-related changes in neurons of the abducens nucleus could be a consequence of the high degree of motor activity in the extrinsic eye muscles during both the waking and sleeping states.