The cationic amino acid L-arginine is essential for the efficient function of human T cells. The absence of L-arginine, a condition found in tumor microenvironment and inflamed tissues, leads to hyporesponsiveness of human T lymphocytes. This manifests in decreased cytokine secretion and a down-regulation of their proliferation, resulting in the arrest in the G0-G1 phase of the cell cycle. Given the importance of L-arginine for T cell function, the transporter responsible for the transmembrane transport of this amino acid must play a crucial role in T cell function. We thus wondered which arginine transporters are expressed in primary human T cells and if they are regulated upon stimulation. For that reason we examined the expression of all known human L-arginine transporter in stimulated and unstimulated T cells. In addition we studied the influence of arginine availability on the transporter expression. Reverse transcription and subsequent quantitative real time PCR experiments revealed a notable expression of y+LAT-2 and cationic amino acid transporter (CAT) 3, but only a weak hCAT-1 mRNA expression in resting T cells. Upon stimulation with anti-CD3/CD28 coupled beads a distinct hCAT-1 expression occurred, which surpassed the expression of the other transporters. In addition, the observed hCAT-1 mRNA level was even higher in arginine-depleted conditions. We also monitored a strong increase in hCAT-1 protein upon T cell stimulation, which peaked after 48 hours of stimulation. These results lead us to assume, that hCAT-1 is primarily responsible for influx of L-arginine in activated T cells. To investigate the subcellular localization of hCAT-1 under different conditions, cell surface expression of hCAT-1 was analyzed in biotinylation experiments. The assays revealed that the upregulated hCAT-1 protein in the absence of L-arginine resulted also in a higher abundance of the transporter in the plasma membrane. Interestingly, the proportion of hCAT-1 detected in the plasma membrane was comparable in cells stimulated in the absence or the presence of L-arginine. hCAT-1 activity was analyzed by the uptake of [3H]-L-arginine (100 µM) into unstimulated and for 48 h stimulated T cells in either arginine-containing or -deficient medium. The experiments revealed a strong induction of L-arginine transport in stimulated compared to unstimulated T cells in both, cells stimulated for 48 hours in the presence and absence of L-arginine. Additionally, transport was completely inhibited by the irreversible CAT-inhibitor N-ethylmaleimide (200 µM). In summary our results indicates, that hCAT-1 plays a crucial role in L-arginine transport in stimulated human T cells. This enlarges our current knowledge of T cell function, the adaptive immune system and tumor-associated immune escape.