In order to clarify a short- and auto-feedback control of FSH release from the anterior pituitary, changes in plasma and pituitary FSH and LH were investigated in rats with FSH implanted into the ME of the hypothalamus and into the anterior pituitary. FSH was bioassayed by the Igarashi-McCann method and LH by the ovarian ascorbic acid depletion method. The following results were obtained. 1. Stereotaxic implantation of FSH (NIH-FSH-ovine) into the ME of ovariectomized rats for 7 days significantly decreased plasma FSH levels in each of 3 experiments and significantly reduced pituitary FSH in 2 out of 3 experiments. Neither pituitary content nor plasma levels of LH showed any significant change in the same rats. 2. Stereotaxic implantation of FSH into the anterior pituitary for 7 days inovariectomized rats did not induce any constant change in plasma FSH levels, butsignificantly decreased pituitary FSH content in all 3 experiments. In 2 out of 3experiments, there was also a significant decrease in plasma LH levels. Pituitary LHcontent remained unchanged. 3. Stereotaxic implantation of FSH into the ME of intact adult female rats for 7 days significantly decreased both plasma and pituitary FSH, but the plasma and pituitary LH did not change significantly. 4. These results clearly demonstrate two different mechanisms of FSH secretion control: the short-feedback and the auto-feedback. The term ‘short-feedback control’ is used to characterize the ability of FSH to inhibit both FSH release and synthesis at the hypothalamic level, while the term ‘auto-feedback control’ is reserved to characterize the ability of FSH to inhibit its own synthesis at the anterior pituitar