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Rabbit antisera to rat pituitary follicle-stimulating hormone (FSH) and to rat luteinizing hormone (LH) were used, in an immunocytochemical probe, to determine the ontogeny and distribution of immunoreactive, endogenous, intraovarian FSH and LH in immature rats. Ovaries from rats 4, 8, 12, and 21 days of age were studied. Both gonadotrophins were first immunodetectable on Day 8. In reactive primordial follicles, LH was restricted to the cytoplasm and nuclei of the surrounding follicle cells. In those follicles possessing both squamous and cuboidal follicle cells, i.e., transitional between primordial and primary, LH was found in both the cytoplasm and nuclei of both follicle cell types. In primary follicles, LH was no longer present in granulosa cells but was concentrated in germ cell cytoplasm. In contrast, in primordial follicles, FSH was restricted to the germ cell but was present in both the oocyte cytoplasm and germinal vesicle. In transitional and primary follicles, FSH remained within the oocyte cytoplasm and germinal vesicle but also became detectable within the cytoplasm and nuclei of granulosa cells. These findings raise some important new questions regarding the role(s) of the gonadotrophins in early follicular development.