The effect of large doses of commercial hCG on thyroid function was studied in eight men who received 100,000 or 150,000 IU hCG iv. These large doses of hCG produced definite thyroidal iodine release (TIR) responses in all eight men. The TIR after hCG administration was more delayed and of lesser magnitude than the TIR responses to TSH and TRH. There were no significant changes in serum T4, T3, or TSH for 48 h after hCG administration. No clinical side effects were noted in the subjects after iv administration of these large doses of hCG. The results of this study indicate that hCG is a weak thyroid stimulator in man.