The reciprocal effects of loading amounts of thyroxine (T4) or triiodothyronine (T3) on the in vivo deiodination of their 125I-labelled isotopes by the rat were studied. It was found that the deiodination of a tracer amount of [125I]T3 as measured by the 24-h urinary 125iodide was significantly decreased after administration of a loading dose of non-radioactive T4. A similar decrease in deiodination of [125I]T3 was observed following the injection of a T3 load. In experiments in which the deiodination of [125I]T4 was assessed, the effects on this of loading doses of non-radioactive T4 or T3 differed; while stable T4 induced a significant decrease in deiodination of [125I]T4, stable T3 was without effect. The data is in agreement with the view that T4 and T3 share tissue deiodinating sites and that in such a system T4 appears to have a stronger avidity for those sites.