Levels of thyrotrophin were determined in the anterior pituitary glands and blood sera of thyroidectomized rats after treatment with triiodothyronine or thyroxine for two weeks. The effects of increasing doses of both hormones on pituitary TSH levels appeared to be of a biphasic nature: whereas lower doses caused an increase, higher doses did so to a smaller extent or even caused a decrease, while at the same time progressively depressing the serum TSH levels. The highest pituitary levels found in thyroid hormone treated thyroidectomized rats were similar to those found in untreated intact rats, and equal doses of thyroid hormone were necessary to normalize the increased serum levels as well as the decreased pituitary contents in thyroidectomized animals. These findings are interpreted as indicating that in thyroidectomized rats the rate of release of TSH is depressed by lower doses of triiodothyronine and thyroxine than the rate of synthesis. Triiodothyronine was about three times more active than thyroxine.