1. Fourteen days after hypothyroidism was induced either by propylthiouracil (PTU) treatment or by thyroidectomy, the serum thyrotropin (TSH) responses to morphine (5 or 20 mg/kg bw), ether stress (30 min) and cold exposure (60 min) were compared with those in normal rats. 2. The decrease in serum TSH levels after morphine and ether stress found in the normal rats were abolished or much reduced respectively. 3. The increase in serum TSH in response to cold exposure and the diurnal rhythm of serum TSH (lower level at night) were also absent in the hypothyroid rat. 4. The stimulating effects of low dose of thyrotropin releasing hormone (TRH) and the inhibitory effects of somatostatin and apomorphine were completely abolished, while the stimulating effects of a high dose of TRH were much reduced in the hypothyroid rat. 5. These results indicate that in the hypothyroid rat the effect of a lack of negative feedback action of thyroid hormone predominates, and that hypothalamic factors are probably unimportant in the regulation of TSH secretion.