Embryonic testis development requires the morphogenesis of cords and growth of all cell populations to allow organ formation. It is anticipated that coordination of the growth and differentiation of various cell types involves locally produced growth factors. The current study was an investigation of the hypothesis that transforming growth factor-α (TGF-α) is involved in regulating embryonic testis growth. TGF-α has previously been shown to function in the postnatal testis. TGF-α and other members of the epidermal growth factor (EGF) family act through the epidermal growth factor receptor (EGFR) to stimulate cell proliferation and tissue morphogenesis. To understand the potential actions of TGF-α in the embryonic testis, general cell proliferation was investigated. Characterization of cell proliferation in the rat testis throughout embryonic and postnatal development indicated that each cell type has a distinct pattern of proliferation. Germ cell growth was transiently suppressed around birth. Interstitial cell growth was high embryonically and decreased to low levels around birth. A low level of Sertoli cell proliferation was observed at the onset of testis cord formation. Sertoli cell proliferation in early embryonic development was low; the levels were high later in embryonic development and remained high until the onset of puberty. Both TGF-α and the EGFR were shown to be expressed in the embryonic and postnatal rat and mouse testis. Perturbation of TGF-α function using neutralizing antibodies to TGF-α on testis organ cultures dramatically inhibited the growth of both embryonic and neonatal testis. TGF-α antibodies had no effect on cord formation. The TGF-α antibody was found to be specific for TGF-α in Western blots when compared to EGF and heregulin. Testis growth was also inhibited by perturbation of EGFR signaling using an EGFR kinase inhibitor. Therefore, TGF-α appears to influence embryonic testis growth but not morphogenesis (i.e., cord formation). Treatment of embryonic testis organ cultures with exogenous TGF-α also perturbed development, leading to an increased proliferation of unorganized cells. Testis from EGFR and TGF-α knockout mice were analyzed for testis morphology. TGF-α knockout mice had no alterations in testis phenotype, while EGFR knockout mice had a transient decrease in the relative amount of interstitial cells before birth. Observations suggest that there may be alternate or compensatory factors that allow testis growth to occur in the apparent absence of TGF-α actions in the mutant mice. In summary, the results obtained suggest that TGF-α is an important factor in the regulation of embryonic testis growth, but other factors will also be involved in the process.