The binding and internalization of 3,3'-[125I] 5-triiodo-L-thyronine ([125I]T3) was studied in cultured Swiss 3T3-4 mouse fibroblasts. At 0 C, the binding of T3 to cells is saturable, reversible, and stereospecific. These results together with those of earlier fluorescence studies using rhodamine-labeled T3 demonstrate the presence of specific plasma membrane T3 receptors. At 37 C, the uptake of T3 reached a steady state after 1 h, and approximately 57 fmol T3 were specifically taken up by 10(6) cells. In other cell lines, 7, 19, and 201 fmol T3 were specifically taken up by Chinese hamster ovary cells (subclone 10001), Kirsten sarcoma virus-transformed NIH 3T3 mouse fibroblasts, and nontransformed NIH 3T3 mouse fibroblasts, respectively. Incorporation of T3 into nuclei followed similar kinetics and accounted for approximately 9% of the total cellular uptake. Equilibrium binding studies of T3 to isolated nuclei showed one class of binding sites with an apparent association constant of 5 X 10(9) M-1 and a binding capacity of 16 fmol/100 micrograms DNA. At 37 C, the internalization of T3 was nearly totally blocked by antimycin A or rotenone, inhibitors of oxidative phosphorylation. These results indicate that the uptake of T3 is an energy-dependent process. In the presence of bacitracin or monodansylcadaverine, substances that inhibit the receptor-mediated endocytosis of alpha 2-macroglobulin, the cellular uptake of T3 as well as the nuclear incorporation of T3 were inhibited in a concentration-dependent manner. The half-maximal inhibitory concentrations for the cellular uptake of T3 were 90 and 660 microM for monodansylcadaverine and bacitracin, respectively; for nuclear incorporation, they were 70 and 350 microM for monodansylcadaverine and bacitracin, respectively. These results indicate that receptor-mediated endocytotic uptake of T3 is a physiologically significant pathway.