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Single subcutaneous injections of varying doses of estradiol benzoate (0.2–5 µg) in ovariectomized female rats were capable of lowering plasma FSH and LH and elevating plasma prolactin as determined by radioimmunoassay. The effects were demonstrable by 24 h and more marked at 48 h. Once an effect had been obtained, there was no clearly increased effect of higher doses of steroid. Pituitary content of FSH, LH, and prolactin increased significantly at 48 h with the 5 µg doses of estrogen, accompanied by an increase in anterior pituitary weight. Hypothalamic content of FSH-releasing factor (FRF) was reduced at 48 h only by the 1 µg dose of estradiol, whereas the content of LH-releasing factor (LRF) declined at 48 h only with the 0.2 and 0.5 µg doses. Prolactin-inhibiting factor (PIF) declined at 24 h after 0.5, 1, or 5 µg of estradiol and the level was still low at 48 h in the case of the 1 or 5 µg doses. The results are interpreted to mean that single injections of estradiol can inhibit release and synthesis of FSH and LH at least in part by a hypothalamic mechanism. With high doses, an elevation in stored hormone results in the pituitary, since synthesis of hormone is inhibited less than release. On the other hand, estrogen increases release and synthesis of prolactin and this effect is achieved, at least in part, by altered hypothalamic function.

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