Estradiol (E2) and estrone (E1) were extracted from the sera of normal (NF) and androgenized (AF) female rats of various ages, separated by column chromatography, and quantiated by immunoassay. The AF received 50 µg of testosterone propionate on the 5th day postpartum. Serum gonadotropins (FSH, LH, prolactin) were also measured by radioimmunoassay. AF had 3 times more E2 and E1 than did NF at 10 days of age. The concentration of both estrogens decreased linearly with age between days 10 and 20 in NF. AF showed a 50 % reduction in E2 and a 77% reduction in E1 between days 10 and 15, but both increased greatly again by day 20; they were then 15 times the concentrations in NF. In AF older than 40 days the serum estrogens were at the same level as those in NF on the morning of proestrus. FSH and LH levels of AF were only 50 % of those in NF at day 10; they were also higher in NF on day 15. At 20 and 25 days, however, the FSH and LH differences disappeared. Prolactin concentrations were similar in AF and NF at 10, 15, 20 and 25 days of age, but by day 30 the concentration was higher in AF. Prolactin concentrations increased with age in the AF. The results support the hypothesis that estrogens play a role in producing the hypothalamic changes in AF. Furthermore, serum gonadotropin levels may be involved in androgenization because they are much lower in AF than in NF.