Chronic treatment of C3H mice with thyroxine for 5-14 days was no more effective than hypophysectomy in depressing the thyroid iodide trap. Thyroid stimulating hormone (TSH) had some stimulating effect on the activity of the thyroid iodide trap. The activity of the mouse thyroid iodide trap is much greater than that of the rat and either thyroxine or hypophysectomy are ineffective in completely depressing the iodide trap. The combination of TSH and propylthiouracil caused a greater response, but insufficient to serve as an assay technique for thyrotrophin.