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The potentiation of the propylthiouracil (PTU)-induced goitrogenesis after chronic administration of small doses of thyroid hormone has been attributed to the high circulating level of thyrotrophin (TSH) or to the re-instatement of insulin. In re-examining this problem radioimmunoassayable concentrations of TSH, thyroxine (T4), insulin, and growth hormone (GH) were observed in sera of rats at sequential intervals after surgical or chemical thyroidectomy and after thyroidectomy and replacement therapy with GH or T4. In addition, TSH, GH or a combination of both hormones were injected into hypophysectomized recipients in a further attempt to delineate the effect of either hormone on the thyroid. As expected, the rate of body growth was inversely proportional to the apparent severity of the hypothyroidism achieved in the several experimental groups. Goitrogenesis was enhanced after T4 treatment but evidently was not the exclusive result of increased blood levels of TSH or insulin. Evidence is presented that suggest the enhancement of goitrogenesis may be a growth phenomenon involving the additive or synergistic action of GH and TSH and possible of other hormones.