The pituitary content of fsh was equivalent to approximately 50 μg nih. fsh sl/gland in female hamsters aged 26 days and fell to 12 μg/gland by 40 days. Male hamsters had similar levels at 26 days but the hormone content rose to 105 μg/gland by 40 days. This contrast between males and females is similar to that found in rats during sexual maturation. In guinea-pigs, pituitary fsh fell in both males and females from means of 45 and 40 μg/gland at 18 days to 15 and 12 μg/gland at 88 days. Testosterone treatment of female rats on the 5th day of life reduced pituitary fsh to 65% of normal at 28 days of age but caused no significant changes at 22, 25, 34 or 40 days.