Subcutaneous administration of thyroxinc and some of its analogues to chronically thiouracilized rats revealed the following calorigenic potencies expressed as percentages of corresponding potencies in normal adult rats: L-thyroxine 7% 3,5,3′-triiodo-L-thyronine, 3,5,3′,5′-tetraiodinatcd thyroacotic and thyropropionic acids, and a soluble thyroglobulin preparation, 29–43%3,5,3′-triiodinated thyroacetic and thyropropionic acids, about 100%. Further tests in normal and thyroidectomized adult rats, half of which received thiouracil, showed that the marked reduction in calorigenic effectiveness of thyroxinc occurred in both types of the thiouracil-treated animals. The activity of triiodothyropropionic acid in thiouracilized rats was not significantly different from that observed in corresponding normal and thyroidectomized controls. The thiouracilized rats were found to be excreting, in their feces, approximately 2.3 times (p <0.01) as much antigoitrogenically-active material as the nonthiouracilized rats, al...