ExtractFollowing weaning, Sprague Dawley rats were given 60% of a normal Purina chow intake until 7 weeks of age. There was a reduced DNA content in the cerebrum at 49 days of age. Previous failure to demonstrate reduced DNA content in rat brain during undernutrition could relate to preoccupation with whole brain analysis. The present study indicates that cerebral DNA increases in normal rats after weaning.The cell size or the ratio of protein: DNA increased in the muscle tissues of rats subjected to the caloric restriction, while the ratio of RNA: DNA increased in all tissues studied. These findings are contrary to those found in protein deficiencyper se.Carcass weight, fat, and water were below expected levels, but skeletal collagen was less affected.Rats from 26 to 38 days of age, given an increased insulin-induced caloric intake, showed an excess weight gain per gram of food consumed per day and an excess growth of muscle and fat. In contrast, cerebral weight, water, protein, DNA, and RNA content were reduced possibly because of periodic hypoglycemia.SpeculationFollowing weaning, rats subjected to sustained caloric restriction have reduced cell numbers for age and are said not to reach expected cell populations on rehabilitation. With caloric restriction, growth hormone may no longer be effective at the cellular level, but insulin activity continues. Although the ratio of cytoplasm: nucleus is maintained, DNA replication is minimal. Alternatively, since DNA content in the cerebrum decreases, hypothalamic or acidophilic cells may be lost, and growth hormone production may be insufficient for adequate DNA replication and somatic growth.When protein is restricted, the failure of cells to increase in size may be associated with atrophy of the pancreas and a decrease in insulin production.