The uterine weight of hypophysectomized immature female rats receiving 20 I. U. of chorionic gonadotropin daily was used to assay serum FSH content. The sera of intact adult male and female rats, and gonadectomized male and female rats, some treated with estradiol benzoate (2 [mu]g daily) and testosterone pripionate (100 jug daily), were assayed. FSH was detected in the serum of the gonadectomized rats, but not in intact rats. Castrated males had a greater serum FSH content than spayed females. Both estradiol and testosterone decreased serum FSH in the castrated males, but only estradiol was active in the spayed females.