To assess the consequences of prolonged exposure to elevated levels of LH/human chorionic gonadotropin (hCG) in the female, we developed a transgenic (TG) mouse model (hCGβ+) that overexpresses the hCGβ-subunit cDNA. Because of the promoter used, ubiquitin C, the transgene is expressed in multiple tissues, including the pituitary gland, in which coupling with the endogenous common α-subunit results in synthesis of high levels of bioactive hCG. The TG females presented with precocious puberty, infertility, enhanced ovarian steroidogenesis, and abnormal uterine structure. Pituitary enlargement was evident from the age of 2 months, which progressed to adenomas by the age of 10–12 months. Immunohistochemical studies and electron microscopy demonstrated lactotrope origin for the adenomas, associated with severe hyperprolactinemia. The mammary glands of TG females showed marked lobuloalveolar development followed by mammary tumors with characteristics of adenocarcinoma at the age of 9–12 months. More than 90% of penetrance and high frequency of metastasis (47%) was observed. Formation of the pituitary and mammary gland tumors was totally abolished by ovariectomy despite persistently elevated hCG levels. Taken together, these findings suggest that the hCG-induced aberrations of ovarian function are clearly responsible for the extragonadal tumors observed in these TG mice.