A tracer dose of [125I]T3 was given iv to normal, thyroidectomized, and propylthiouracil-fed rats and the distribution of radioactivity in serum and liver fractions was studied over 1 h. Total liver homogenate and serum 125I were higher at all times in hypothyroid rats and, in all groups, showed a continuous fall over the period studied. Hepatic nuclear 125I was maximal at 20 min in all and was greater in hypothyroid rats; there was more 125I in the hepatic cytosol of normal rats than in that from either thyroidectomized or propylthiouracil-fed animals. Binding studies with [125I]T3 and purified hepatic neclear preparations in vitro indicated that both the association constant, Ka (1.08-9.0 x 10(9) M-1) and the capacity (500-600 pg/mg DNA) in thyroidectomized and goitrogen-treated rats were similar to those obtained with normal animals. Cytosol, on the other hand, showed a decrease in binding capacity without change in affinity in livers of hypothyroid rats. Analysis of binding data by Hill plots indicated the presence of both positive and negative cooperativity in binding of T3 by rat liver cytosol proteins. In the in vitro experiments, higher serum radioactivity alone could not account for increases in the hepatic nuclear 125I in the hypothyroid rats because cytosol 125I (presumably in dynamic exchange with both blood and nuclei) was less. Consequently, cytosol T3-binding proteins may regulate the free T3 concentration in the cell and, thus influence the distribution of the hormone in other cellular compartments.