A study of the changes in plasma LH and FSH as measured by radioimmunoassay (RIA) has been done in maturing male and female rats. The LH concentration patterns as a function of age (5 to 45 days) were similar in the two sexes. On the contrary, the FSH concentration patterns were noticeably different. The plasma FSH levels in female rats peaked at around IS days of age and subsequently decreased exponentially. No comparably marked changes were seen in plasma FSH in the male. Hemigonadectomy at 10 days of age induces a rapid response in plasma gonadotropin levels in males. This is especially marked with FSH for which plasma levels increased to 240% over control value 2 days after surgery. In contrast to the male, the female responds with a small increase in FSH levels but not LH and the response has a delay of at least 10 days-post-hemigonadectomy. It is concluded that the control system for gonadotropins in the male is under the influence of steroids at very early ages; in the female, the ovarian steroids do not appear to modify the secretion of FSH until the animals are 20 days old.
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