A comparison of the amount and distribution of lipofuscin pigment in neurons of cerebral cortex in prenatally X-irridiated (90 r) and control aged rats was carried out in areas 3 and 17. The proportion of the neuron somata occupied by the pigment bodies, as determined through the application of the ‘random-hit’ principle in phase-fluorescence microscope studies, was not significantly different in irradiated and control brains in either cortical area. However, the amount of pigment was greater in neurons of area 3 than those of area 17 in both irradiated and control animals. Ultrastructural studies of cerebral cortical tissues in area 3 revealed no abnormalties in neurons or glia of irradiated as compared with control rats.