At least three forms of phenol sulfotransferase (ST) ST1B1, ST1A1 and ST1C1 are contained in rat livers. To identify the form contributing to the metabolism of 3,3',5-triiodothyronine (T3), functional characterization of these forms was performed by expression in Escherichia coli. ST1B1 and ST1C1 were shown to be active on sulfation towards T3 with high affinity (Km: 44.4 and 25.8 microM, respectively), whereas ST1A1 had low affinity. In Western blotting using antibodies raised against the individual ST, hepatic contents of each ST were quantitatively determined. ST1B1 showed no clear sex-difference, whereas the level of ST1C1 was higher in adult males than adult females. The content of ST1B1 was 1.4, 6.8 and 10 times higher than that of ST1C1 in adult males, adult females and both sexes of immature rats, respectively. The developmental pattern of ST1B1 was similar to that of ST1A1, but differed from that of ST1C1. These results indicate that ST1B1 and ST1C1 are involved in T3 metabolism in rats and ST1B1 is the constitutive form across sexes and ages.