We studied the effects of TSH on rat thyroid ornithine decarboxylase (ODC) activity. After 1 day of goitrogen treatment, there was an abrupt fall in serum triiodothyronine (T3),a rise in circulating TSH, and a dramatic increase in thyroid ODC activity. Despite the continued rise in TSH and progressive increase in thyroid gland size with further treatment, thyroid ODC activity declined on the third day and remained at submaximal levels. Thyroid ODC activity was also stimulated in a dose-related manner by administration of exogenous TSH. Little TSH effect was noted before 3 h. Maximal ODC activity occurred between 4 and 5 h. The TSH stimulation of ODC could be inhibited by pretreatment with actinomycin D or cycloheximide, suggesting that the increase in ODC activity requires new RNA and protein synthesis. Although pretreatment with agents that alter microtubule structure (e.g., colchicine and vinblastine) prevent stimulation of ODC activity by TSH, additional data suggest, but do not confirm, that hormone secretion and ODC activation may be dissociable. Further studies were undertaken to determine whether cyclic AMP (cAMP) or prostaglandins playedany role in the regulation of thyroidal ODC activity. Dibutyryl cAMP, alone, or together with aminophylline, did not stimulate thyroidal ODC activity in dosages which concomitantly stimulated adrenal enzyme activity. Likewise, prostaglandin E2 (PGE2) did not stimulate thyroidal ODC activity,) but did stimulate adrenal enzyme activity in a dose-related manner. However,pre-treatment of rats with inhibitors of prostaglandin synthesis prevented the activation of thyroidal ODC by TSH. One inhibitor, indomethacin, attenuated the TSH stimulation of enzyme activity ina dose-related manner. Indomethacin pretreatment also resulted in approximately a 10–fold decreasein thyroidal pros-taglandin levels. Exogenous (PGE2) in dosages as high as 500 µg, did not overcome the inhibitory effect of indomethacin on ODC activation. Although the preciserole for endogenous prostaglandins remains to be defined, it does appear that a reduction in thyroidal prostaglandins preventsactivation of the enzyme by TSH. (Endocrinology96: 1403, 1975)