Ovarian primordial follicle reserve is considered hormonally independent or subject to depletion by FSH-driven follicle recruitment. To explore specific in vivo effects of FSH on early follicle populations in the absence of luteinizing hormone (LH) activity, we examined mature hypogonadal (hpg), gonadotrophin-deficient mice expressing transgenic (tg) human FSH. Sustained expression of tg-FSH (5.3 +/- 0.3 IU/l) increased ovary weights fourfold and significantly elevated total primordial follicle numbers twofold in tg-FSH hpg (4209 +/- 457) relative to non-tg hpg (2079 +/- 391) and wild-type (2043 +/- 195) age-matched ovaries. Absolute primary follicle numbers in tg-FSH hpg ovaries were similar to non-tg hpg and wild-type ovaries. Furthermore, tg-FSH quantitatively increased secondary and antral follicles in hpg ovaries to numbers equivalent to wild-type, but did not induce ovulation, indicating a selective FSH response without LH. Circulating inhibin B and inhibin A levels were significantly increased in tg-FSH hpg females compared with hpg controls, and inhibin B correlated with antral number, consistent with FSH-driven antral follicle formation. These findings revealed that sustained pituitary-independent FSH activity, in the absence of endogenous gonadotrophins, promotes an increase in primordial follicle reserve despite also stimulating follicular growth in mature females. Therefore, the tg-FSH hpg ovary presents a novel paradigm to evaluate specific gonadotrophin effects on follicle reserve and recruitment.