Transgenic (TG) mice, expressing the Simian Virus 40 T-antigen (Tag) under a 6-kb fragment of the murine inhibin alpha-subunit promoter (inh alpha p), develop gonadal tumors of granulosa/theca or Leydig cell origin. We showed previously that adrenocortical tumors develop if the TG mice are gonadectomized but never develop in intact animals. However, if functional gonadectomy was induced by GnRH antagonist treatment or by cross-breeding the TG mice into the hypogonadotropic hpg genetic background, neither gonadal nor adrenal tumors appeared. Since the most obvious difference between the gonadectomized and GnRH-antagonist-treated or Tag/hpg double mutant mice is the elevated gonadotropin secretion in the first group, we examined whether the adrenal tumorigenesis would be gonadotropin-dependent. Surprisingly, both the adrenal tumors and a cell line (C alpha 1) derived from one of them expressed highly functional LH receptors (LHR), as assessed by Northern hybridization, immunocytochemistry, ligand binding, and human CG (hCG)-stimulated cAMP and steroid production. No FSH receptor expression was found in the adrenal tumors by RT-PCR. hCG treatment of the C alpha 1 cells stimulated their proliferation, as measured by [3H]thymidine incorporation. This effect was related to hCG-stimulated steroidogenesis since progesterone, testosterone, and estradiol, at physiological concentrations, also stimulated the C alpha 1 cell proliferation. Different adrenocortical cells expressed initially LHR and Tag, whereas both were highly expressed in the tumor cells. In conclusion, the high level of functional LHR in the adrenal tumors indicates that this receptor can function as tumor promoter when ectopically expressed and stimulated by the ligand hormone.