We have postulated the existence of a thyroid hormone (T3 or T4)-mediated short loop feedback mechanism in thyroid based on studies wherein T3 or T4 inhibited TSH-induced ornithine decarboxylase (ODC) activity in both intact and hypophysectomized rats (J Clin Invest 57: 745, 1976). Since it has been suggested that some or all of this inhibition may be secondary to T3-- or T4--induced suppression of endogenous TSH, it was felt to be of interest to perform further studies. In contrast to our initial findings, thyroid ODC in the hypophysectomized rat was generally unresponsive to exogenous TSH (2 U) administration. A normal response could be restored by prior injection of a low dose (0.2–1 U) of TSH, which did not influence thyroid ODC activity per se, 16 h before administration of the 2-U TSH test dose. Similarly, pretreatment with dibutyryl cAMP (4–15 mg) plus aminophylline (5 mg) at -16 h was also effective in restoring the response to exogenous TSH (2 U) to near normal levels. T3 pretreatment effectively inhibits TSH-induced thyroid ODC activity in intact rats. However, when intact rats were pretreated with 1 U TSH together with T316 h before administration of the 2-U TSH test dose, no inhibition of ODC activity was observed. Additionally, when hypophysectomized rats were pretreated at -16 h with TSH (0.5–1 U) together with T3 or T4, no inhibition of TSH-induced ODC was seen. Goitrogen treatment increases rat thyroid cAMP-dependent protein kinase activity (CNDPK) by 60% over control, whereas addition of T4 to the drinking water (5 mg/liter) decreases the activity by 35%. Thyroid CNDPK was 50% lower in hypophysectomized rats than in intact rats; pretreatment of hypophysectomized rats with TSH restored the CNDPK levels to near normal. Goitrogen treatment increases rat thyroid adenylate cyclase activity by 130% over control, whereas hypophysectomy lowers basal activity by 35%. It was concluded that TSH exerts a tonic regulatory effect on thyroid function and the previously reported inhibition of TSHinduced ODC by T3 or T4 in intact rats may, totally or in part, have been due to endogenous TSH suppression.