Studies were undertaken to define the concentration of the high affinity NADPH-dependent cytosolic 3,5,3'-triiodo-L-thyronine (T3)-binding protein (CTBP) in various tissues of adult rats and in those of developing rats. The maximal binding capacity (MBC) for T3 binding was calculated from the data obtained by Scatchard analysis of T3 binding to the charcoal-extracted cytosol in the presence of 50 microM NADPH. There were no significant differences in the affinity constant among various tissues, whereas the MBC was different among tissues in adult rats. When the levels of MBC were expressed by moles/DNA concentration, the order was as follows; kidney greater than heart greater than or equal to cerebrum greater than or equal to liver much greater than testis greater than cerebellum greater than spleen. The CTBP was not detected before birth in liver, heart, and spleen. The CTBP in these tissues emerged 5 days after the birth, and increased during 6 weeks after the birth. The CTBP in kidney was not detected before birth but was detected at the time of birth. The level of MBC in kidney increased during 3 weeks after the birth. In contrast, the CTBP was detected 5 days before birth, and the level of MBC increased at the time of birth both in cerebrum and cerebellum. The MBC in cerebrum gradually decreased after birth, but began to increase again 2 weeks after the birth. The level of MBC in cerebellum did not increase again. These results suggested that the target tissues of thyroid hormone are enriched with NADPH-dependent CTBP, and that the changes in the concentration of the CTBP are related to the growth of tissues. It was speculated that the CTBP has a specific function in cerebrum at the time of birth although the action of the CTBP is not certain.