Transforming growth factor beta 1 (TGF1) is a multifunctional cytokine implicated in gonad and secondary sex organ development, steroidogenesis and spermatogenesis. To determine the physiological requirement for TGF1 in male reproduction, Tgfb1 null mutant mice on a Prkdc^scid immunodeficient background were studied. TGF1 deficient males did not deposit sperm or induce pseudopregnancy in females, despite an intact reproductive tract with morphologically normal penis, seminal vesicles and testes. Serum and intra-testicular testosterone and serum androstenedione were severely diminished in TGF1 deficient males. Testosterone deficiency was secondary to disrupted pituitary gonadotrophin secretion, since serum LH and to a lesser extent serum FSH were reduced, and exogenous LH replacement with hCG induced serum testosterone to control levels. In the majority of TGF1 deficient males, spermatogenesis was normal and sperm were developmentally competent as assessed by in vitro fertilization. Analysis of sexual behavior revealed that while TGF1 null males showed avid interest in females and engaged in mounting activity, intromission was infrequent and brief, and ejaculation was not attained. Administration of testosterone to adult males even after neonatal androgenization was ineffective in restoring sexual function, however erectile reflexes and ejaculation could be induced by electrical stimulation. These studies demonstrate the profound effect of genetic deficiency in TGF1 on male fertility, implicating this cytokine in essential roles in the hypothalamic-pituitary-gonadal axis and in a testosterone-independent regulation of mating competence.