We investigated the characterization and the regulation of TAUT (taurine transporter) and CDO (cysteine dioxygenase), one of the key enzymes of taurine biosynthesis, in human hepatoblastoma HepG2 cells. The activity of TAUT in the HepG2 cells was evaluated by means of a sodium- and chloride-dependent high-affinity transport system, the characteristics of which were similar to those of the beta amino-acid-specific taurine transport system described previously for various tissues [Uchida, Kwon, Yamauchi, Preston, Marumo and Handler (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 8230-8234; Ramamoorthy, Leibach, Mahesh, Han, Yang-Feng, Blakely and Ganapathy (1994) Biochem. J. 300, 893-900; and Satsu, Watanabe, Arai and Shimizu (1997) J. Biochem. (Tokyo) 121, 1082-1087]. By culturing in a hypertonic medium, the intracellular taurine content of HepG2 cells was markedly increased. Under hypertonic conditions, the activity of TAUT was up-regulated, and the results of the kinetic analysis suggested that this up-regulation was associated with an increase in the amount of TAUT. The expression level of TAUT mRNA was markedly higher than that of the control cells. The expression level of CDO mRNA was also up-regulated under the hypertonic conditions. Culturing the cells in a taurine-rich medium resulted in both the activity of TAUT and the expression level of TAUT mRNA being down-regulated in HepG2 cells. On the other hand, the expression level of CDO mRNA was not affected under a taurine-rich condition. The present results show that both TAUT and CDO were co-operatively regulated in response to hypertonicity, but did not co-operatively respond to the change in extracellular taurine concentration. Generally, the TAUT and taurine biosynthetic enzymes have independent regulatory systems, but under certain conditions, they could be regulated in harmony with each other.