The action of periodate on FSH [follicle stimulating hormone], ICSH [interstitial cell stimulating hormone], TSH [thyrotropin], growth hormone, HCG [human chorionic gonadotropin] and PMSG [pregnant mare''s serum gonadotropin] has been investigated. With the exception of growth hormone, all the other hormones show markedly diminished activity after treatment with periodate. Additional information is presented on the effect of oxidation with periodate on the biological activities of [alpha]-corticotropin and lactogenic hormone. It is suggested that periodate be employed to decrease the TSH and ICSH activities in growth hormone preparations; these activities arise from contaminants in such preparations. Finally, it has been shown that periodate-treated growth hormone is still capable of partially maintaining the adrenal weights of rats that have undergone hypo-physectomy.