Normal and thyroidectomized rats were treated with propylthiouracil and thyroxine. The intact animals given 2 μg. thyroxine per day exhibited goiter and increased thjTotropin along with greater than normal protein-bound iodine in the scrum. After thyroidectomy, 2 μg. thyroxine per day produced normal concentrations of both thyrotropin and protein-bound iodine; in similar animals treated with propylthiouracil thyrotropin was again normal but protein- bound iodine was twice normal. Propylthiouracil thus appeared to have an extra-thyroidal action on injected thyroxine as indicated by marked increase in protein-bound iodine.