Young SWR mice possessing spontaneous ovarian granulosa cell (GC) tumors were examined for evidence of endocrine dysfunction associated with tumorigenesis. Tissue levels of hormones in tumor host and normal control females were measured by radioimmunoassays, ovarian luteinizing hormone (LH) receptors by uptake of 125I-labeled human chorionic gonadotropin (hCG) administered iv, and ovarian 3-beta-hydroxysteroid dehydrogenase (3-beta-OH) activity by histochemical techniques. When data from tumor host mice were compared with control data, hypothalamic gonadotropin releasing hormone content was not significantly different. Pituitary LH and follicle-stimulating hormone (FSH) contents were significantly decreased. Serum FSH, but not LH, levels were significantly reduced. No specific uptake of 125I-labeled hCG by tumor tissue was detected, whereas uptake by nontumorous contralateral ovaries was identified and found to be similar to that of control ovaries. With respect to serum steroids in tumor host mice, progesterone, dihydrotestosterone, and testosterone were significantly reduced, whereas androstenedione, dehydroepiandrosterone, corticosterone, estrone, and estradiol were normal. Frozen sections of tumor tissue failed to show any 3-beta-OH activity, whereas prominent activity was observed in non-tumorous contralateral and control ovaries. Serum thyroxine levels, evaluated because of the known depressive effects of hypothyroidism on reproductive function, were found to be significantly elevated in tumor host mice. The above results suggest that in SWR mice with spontaneous GC tumors, gonadotropins are moderately suppressed; the granulosa tumor cells do not have LH-hCG receptors; steroidogenesis by tumor tissue is reduced, whereas peripheral conversion of adrenal androgen precursors to estrogens is normal; and elevated serum thyroxine levels have a secondary role in established GC tumors.