Forty-six patients whose thyroid glands had been ablated for thyroid carcinoma, maintained on replacement treatment with triiodothyronine (T3), have been followed by means of serial determinations of pituitary thyrotropin (TSH) levels and of the TSH response to thyrotropin-releasing hormone (TRH), in order to verify the actual dose required to suppress TSH secretion. The treatment was begun with a dose of T3 roughly related to body weight (about 1 mug/kg/day). The daily dose was sequentially adjusted according to the measured TSH levels. The dose of T3 which reduced serum TSH within the normal limits varied from 40 to 120 mug daily. More precisely, it was as low as 40-50 mug daily in five patients and as high as 90-120 mug daily in three patients. The dose, which was given according to their body weights, varied from 0.75 to 1.12 mug/kg/day. TRH administration produced a marked increase of TSH levels in those patients whose TSH was elevated, and varying responses (from no response to a response three times the normal one) in those patients whose TSH was within the normal limits. In the latter group of patients, an increase of the T3 dosage inhibited the TSH response to TRH. These data suggest that, even though 1 mug/kg/day is an adequate T3 dose for the great majority of patients, the extent of TSH suppression is not surely predictable, but must be evaluated case by case by means of serial determinations of TSH levels and of the TSH response to TRH.