The amino acids tyrosine and tryptophan are precursors of physiologically active amines in the central nervous system. To reach the brain they have to compete with other large neutral aminio acids (LNAA) for the normally saturated carrier by which these amino acids are transported into the brain. The beta-adrenergic agonist isoprenaline is demonstrated to cause an increase in the brain concentration of most LNAA without a concomitant decrease in any of them. This finding indicates that the transport of LNAA into the brain is regulated by a beta-adrenergic mechanism.