AbstractThe intracellular level of taurine is maintained both by the taurine transporter (TAUT) and by endogenous synthesis from Met and Cys. We investigated in the present study the regulation of TAUT and of cysteine dioxygenase (CDO), one of the major taurine biosynthetic enzymes, in 3T3‐L1 adipocytes. The TAUT activity, expression of TAUT and CDO mRNA were up‐regulated by hypertonicity. In contrast, the TAUT activity, expression of TAUT and CDO mRNA were down‐regulated by taurine‐rich conditions. Furthermore, it was indicated that the up‐regulation of TAUT activity resulted from the increased number of expressed TAUT, and not by the change in affinity of TAUT. On the other hand, the taurine‐induced down‐regulation of TAUT activity resulted not only from a decrease in the number of expressed TAUT but also from a decrease in their affinity. These results suggest that murine TAUT and CDO were cooperatively regulated in response to hypertonicity and taurine‐rich conditions.