The most abundant basolateral amino acid transporters of kidney proximal tubule and small intestine, LAT2- and y⁺LAT1-4F2hc, function as obligatory exchangers and thus do not directly contribute to the net amino acid (re)absorption. The aromatic amino acid transporter Tat1 (Slc16a10), which is known to work as a low affinity facilitated diffusion pathway, co-localizes in the proximal kidney tubule with the two exchangers mentioned above. Furthermore, TAT1 protein was localized in the basolateral membrane of small intestine enterocytes, in the sinusoidal side of perivenous hepatocyte and in plasma membrane of skeletal muscle fibers. Tat1 transcripts were found also in heart, adipose tissues and brain. Exploiting the Xenopus laevis oocyte we have previously shown that TAT1 can complement the vectorial transport function of LAT2-4F2hc by recycling exchange substrates. Tat1 knockout mice (Ingenium Pharmaceuticals AG) grow normally and show no gross phenotype. When kept in metabolic cages for 24 hours with free access to food and water, they exhibit specific aminoaciduria: their urine contains higher amounts of aromatic amino acids, of Asp and Lys. The analysis of plasma showed increased levels of Tyr and Trp resulting in an almost normal fractional reabsorbtion of most amino acid in the kidney. Preliminary data show a slower appearance of ³H-Phe tracer into the blood after oral administration and an accumulation of 2-I¹²⁵-L-Phe in the kidney 30 min after intravenous injection, suggesting a slower basolateral efflux kinetics. This data suggest (a) a role for TAT1 in net transepithelial (re)absorption of amino acids and their delivery into peripheral tissues and (b) that at least one additional amino acid transporter capable of recycling exchange substrates is expressed in the basolateral membranes of (re)absorbing epithelia. Further investigations will aim at better understanding the role of TAT1 in vivo and its impact on intracellular homeostasis in various tissues.