The administration of insulin or the ingestion of glucose increases the concentration of tryptophan and tyrosine in the brain. This increase is associated with a parallel decrease in the concentration of free tryptophan and tyrosine in serum. The results suggest that insulin enhances the transport of tyrosine and tryptophan from blood to brain. The results, moreover, suggest that exogenous or endogenous insulin enhances the transport of tyrosine and tryptophan from blood to brain.