Rhesus monkeys consumed purified diets that supplied either low or adequate levels of protein (3.8 vs. 13.9% of energy as casein) from birth until approximately 10 yr of age. A subgroup (PN) was born of mothers that also received low or control levels of protein throughout pregnancy. The deprived groups weighed significantly less than corresponding control groups. Weights of the total brain, cerebellum and brain stem were significantly reduced in the PN deprived group. Analysis of variance also indicated that low protein diets produced a significant reduction in cerebral weight. The concentrations of DNA, protein and eight different lipids from seven different sites in the central and peripheral nervous system were not greatly affected by diet. The total content of lecithin and phosphatidylethanolamine was significantly depressed in some parts of the deprived monkey brains. The deficits in brain weight of the PN group (10% for the cerebrum, 13% for the cerebellum and 18% for the brain stem) were very similar to those observed previously in 1-mo-old monkeys born of protein-deprived mothers and fed low protein diets postnatally. On the other hand, monkeys born of adequately nourished mothers and then fed low protein diets from birth up to 12 yr showed no deficit in brain weight despite reduction in body weight to less than half of control values.