Sex steroids control cellular phenotypes by binding to receptor proteins that in turn regulate downstream gene expression. They are important tropic factors in hormonally responsive tissues and have been implicated in the pathogenesis of both benign proliferations and malignancies at some of these sites. Knockout mice lacking inhibins, α:β heterodimeric peptide hormones of the TGFβ superfamily, develop gonadal tumors that produce sex steroids and depend on pituitary gonadotropin hormones. To better appreciate how sex steroid receptor signaling pathways contribute to the loss of granulosa/Sertoli cell proliferation in the ovary and testis of inhibin α (Inhα) knockout mice, we are using both pharmacologic and genetic approaches. Roles of androgens in testicular tumor development have been investigated in our previous studies using double-mutant mice lacking inhibins and carrying the null testicular feminization (tfm) mutation of the androgen receptor. Herein, we report that androgens also participate in the development of ovarian tumors, as tumor development is forestalled in mice treated with flutamide, a nonsteroidal inhibitor of androgen actions. Additionally, we generated double-mutant mice lacking estrogen receptor α (ERα) and Inhα or ERβ and Inhα, as well as triple-mutant mice lacking ERα, ERβ, and Inhα to determine the effects of individual and combined ER signaling pathways on tumor development. Although estrogens may have proliferative effects during follicle development and are important in specifying the granulosa cell phenotype, ERα and ERβ signaling are not essential for timely granulosa cell tumor development or granulosa cell-like morphological features in ovarian tumors. However, redundant ER signaling through ERα and ERβ in males is critical for testicular tumor formation, as triple-knockout, but not double-knockout, males are protected from early Sertoli cell tumorigenesis and death. Together, these studies indicate important and sexually dimorphic functions of estrogens and androgens in tumor development in this mouse model and indicate, for the first time, overlapping functions of ERα and ERβ in Sertoli cell pathophysiology.