Serum levels of FSH in ovariectomized female mice are only partially suppressed by estradiol regardless of the dosage administered. Conversely, within the limits of detection, serum FSH in intact females is totally suppressed by estradiol. Thus, the present experiments used the maximally estrogenized, ovariectomized female as a test system for evaluating other nonestrogenic factors which could depress serum FSH. It was found that a transplanted ovary lowered serum FSH dramatically under such conditions. Of the large number of steroid and pituitary hormones tested, however, only large doses of testosterone propionate yielded any further suppression of serum FSH (24%). Charcoal-extracted follicular fluid (porcine) markedly lowered serum FSH concentrations (50% suppression), while a charcoal-extracted saline homogenate of PMS-treated mouse ovaries also was active in this regard (26% suppression). Finally, charcoal-extracted porcine follicular fluid was administered to nonestrogenized, ovariectomized females, where it both depressed serum FSH and markedly elevated serum LH concentrations. These studies, when viewed in toto, support a contention that follicular fluid contains a nonsteroidal factor which is capable of acting additively with estradiol to regulate FSH secretion. The question of whether this factor also regulates LH secretion in a reciprocal fashion in the mouse requires further work.