The thyrotropic activity of human chorionic gonadotropin (hCG) has been examined in the chick and the rat. Uptake of 32PO4 by chick thyroid increased significantly with injection of bovine thyrotropin (bTSH) with a maximum response at 2.4 mU per chick. On the other hand, no significant stimulation of 32PO4 uptake was detected with injection of graded doses of highly purified hCG up to 0.25 mg per chick. 1 mg of partially purified hCG, equivalent in biological potency to the maximum dose of highly purified hCG used in the chick, did induce a significant increase in 32PO4 uptake. In rats, highly purified hCG stimulated a very significant release (p less than 0.001) of 125I from the thyroid and partially purified hCG had a thyrotropic activity equivalent to 0.42 microU bTSH/U hCG, identical to the value we reported in mice, 0.42 microU bTSH/U hCG. The duration of hCG action on thyroidal release of 125I in the rat was longer than that for bTSH, as it is in the mouse. hCG also induced a significant rise in the serum level of triiodothyronine in rats. We conclude that pure hCG is a weak thyrotropic substance in the rat but not in the chick. These results and other evidence suggest an inhibitory role for the densely glycosylated 30 amino acid residue C-terminal extension on the beta-subunit of hCG which limits, by steric hindrance, the interaction of the TSH-like hCG 'core' with thyrotropin receptors.