The recent cloning of ATA2, a cDNA displaying characteristics identical to the System A transporter, has provided the first molecular tool for study of System A-mediated amino acid transport in liver. Despite the 233 +/- 9 and 472 +/- 11% increase in System A transport activity following partial hepatectomy at 6 and 12 h, respectively, the steady-state level of ATA2 mRNA did not show a corresponding marked increase. Examination of the kinetic properties of System A following partial hepatectomy revealed a K-m of 0.26 +/- 0.04 mM which is consistent with the reported K-m for ATA2. These results indicate that a System A transporter present in regenerating liver and ATA2 are identical, but that the increase in System A activity following partial hepatectomy does not result from an increase in steady-state levels of ATA2 mRNA. These observations suggest that ATA2-mediated transport of amino acids is regulated at the posttranscriptional level.