A follicle-stimulating hormone-producing pituitary tumor was demonstrated in a 28-yr-old eunuchoid-appearing male who presented with bilateral hemianopsia, oligospermia and an enlarged sella turcica. A basal luteinizing hormone of 16.0 mIU/ml (2nd IRP-HMG) was normal, but FSH was persistently elevated, 33.0–64.0 mIU/ml (normal < 0.8–16.0 2nd IRP-HMG). Provocative testing of hypophyseal function revealed absent growth hormone and deficient thyrotropin and corticotropin release, while prolactin increased normally following thyrotropin-releasing hormone (TRH) and decreased appropriately after l-dopa administration. Plasma testosterone was low. Immunofluorescent staining of the resected chromophobe adenoma using an anti FSH-antiserum revealed uniform fluorescence of the tumor cells. In vitro organ culture of three fragments of this pituitary adenoma demonstrated (mean ± sem) FSH and LH release of 311.7 ± 29.0 mIU/24 hr and 25.6 ± 2.0 mIU/24 hr respectively. No immunoassayable GH or TSH was demonstrable but small amounts of prolactin were present. FSH and LH release continued for up to 9 days of incubation. Testosterone and estradiol (50 μg/ml) suppressed the release of FSH into the media by 39.1 ± 10.7% (p < 0.05) and 35.9 ± 4.1% (p < 0.01) respectively. At 1 μg/ml estradiol, but not testosterone, caused a statistically significant decrease of FSH release. Neither steroid affected the release of LH.