Young adult rats were fed an isocaloric diet for 28 days. The diet consisted of either 22% casein, or protein-restricted food consisting of a 5% mixture of soy and whey protein, or a synthetic amino acid mixed diet devoid of the essential amino acids tryptophan, valine, lysine, and threonine. The level of the nervous tissue-specific S-100 protein was quantified immunoelectrophoretically in 9 different brain areas. A marked decrease of S-100 was observed in hippocampus and posterior part of the cerebellar vermis per g wet weight in protein-restricted rats. An additional lowering was noticed in sensory motor cortex per mg soluble protein. This effect was potentiated with the amino acid-deficient diet. The in vitro incorporation of 3H-leucine into cerebellar proteins was slightly decreased in the low-protein-fed rats, and more markedly decreased in those receiving an amino acid-restricted diet.