Abstract:A biochemical method is described for the simultaneous quantitative estimation of unidirectional blood‐brain amino acid influx and protein biosynthesis in individual structures of the rat brain. The method involved a double labeling experiment started by the administration of [14C]carboxyl‐labeled amino acids and terminated 2 min after infusion of3H‐labeled amino acids, each at tracer quantities, the total labeling period being 45 min. Specific radioactivities of14C‐ or3H‐labeled phenyl‐alanine, tyrosine, leucine, isoleucine and valine were determined in plasma and in small brain tissue samples for free amino acids, aminoacyl‐tRNAs and proteins. Amino acids were converted to their corresponding 5‐dimethylamino‐naphthalenesulfonyl (Dns, dansyl) derivatives and separated on HPLC C18reversed‐phase columns isocratically according to a newly developed optimizing procedure. The order of influx values between the neutral amino acids in relation to each other was Leu>Tyr>Ile>Phe>Val in every structure examined. Although aminoacylation of tRNAs was found to proceed to a comparable degree for neutral amino acids in all regions investigated, the specific radioactivity of amino acids attached to tRNAs differed substantially from that in the free amino acid pool, especially for leucine and valine. The results indicate the necessity of aminoacyl‐tRNA determinations for tracer incorporation studies in protein synthesis analysis. Relative protein synthesis rates in the halothane‐anesthetized rat were determined to be 30 and 67–91 pmol total amino acid incorporation/min/mg tissue for white and