Giving weanling rats a low protein diet for 56 days retarded the growth of the brain, whereas giving isoenergic amounts of a high protein diet did not. The low protein diet resulted in higher concentrations of aspartic acid, glycine, alanine and histidine in the plasma and of glycine and histidine in the brain. The concentration of methionine was unchanged in the plasma, but raised in the brain. Isoenergetic amounts of the high protein diet did not change the concentration of plasma amino acids, but resulted in some changes in brain amino acids. On rehabilitation the plasma and brain amino acids of the low protein animals returned to normal.