Capacity of rat liver nuclear triiodothyronine (T3) binding protein (NTBP) for T3 more than quadruples from birth through 50-120 days, whether related to DNA or weight of tissue. Ka for T3 doubles during this interval. T3 content of nuclei nearly parallels capacity. The proportion of receptors occupied by T3 is 0.3 at birth, falling to .16 in old rats. In contrast to these changes in liver content of T3-receptor complex, mitochondrial alpha-glycerophosphate dehydrogenase activity changes relatively less during maturation. Administration of estrogen, testosterone, and dexamethasone to immature female rats did not alter NTBP capacity of affinity.