Brains from the 15- and 21-day-old offspring of rats given a 7-percent casein diet from the 5th day of gestation and through lactation showed no evidence of degenerative changes in the nerve cells or fibres, but modifications in structure were seen in every area examined. Cells generally contained less cytoplasm, and the walls of the myelin sheaths were thinner. Developmental changes were retarded, the cell density was greater and dendritic development reduced. The changes were most marked in the basal ganglia, hypothalamic nuclei, cerebellum, and in nuclei having close links with the cerebellum.