FSH mediates its testicular actions via a specific Sertoli cell G protein-coupled receptor. We created a novel transgenic model to investigate a mutant human FSH receptor (FSHR⁺) containing a single amino acid substitution (Asp567Gly) equivalent to activating mutations in related glycoprotein hormone receptors. To examine the ligand-independent gonadal actions of FSHR⁺, the rat androgen-binding protein gene promoter was used to direct FSHR⁺ transgene expression to Sertoli cells of gonadotropin-deficient hypogonadal (hpg) mice. Both normal and hpg mouse testes expressed FSHR⁺ mRNA. Testis weights of transgenic FSHR⁺hpg mice were increased approximately 2-fold relative to hpg controls (P < 0.02) and contained mature Sertoli cells and postmeiotic germ cells absent in controls, revealing FSHR⁺-initiated autonomous FSH-like testicular activity. Isolated transgenic Sertoli cells had significantly higher basal (∼2-fold) and FSH-stimulated (∼50%) cAMP levels compared with controls, demonstrating constitutive signaling and cell-surface expression of FSHR⁺, respectively. Transgenic FSHR⁺ also elevated testosterone production in hpg testes, in the absence of circulating LH (or FSH), and it was not expressed functionally on steroidogenic cells, suggesting a paracrine effect mediated by Sertoli cells. The FSHR⁺ response was additive with a maximal testosterone dose on hpg testicular development, demonstrating FSHR⁺ activity independent of androgen-specific actions. The FSHR⁺ response was male specific as ovarian expression of FSHR⁺ had no effect on hpg ovary size. These findings reveal transgenic FSHR⁺ stimulated a constitutive FSH-like Sertoli cell response in gonadotropin-deficient testes, and pathways that induced LH-independent testicular steroidogenesis. This novel transgenic paradigm provides a unique approach to investigate the in vivo actions of mutated activating gonadotropin receptors.