The TGFbeta superfamily ligand, activin A controls juvenile testis growth by stimulating Sertoli cell proliferation. Testicular levels are highest in the first postnatal week when Sertoli cells are proliferating and spermatogonial stem cells first form. Levels decrease sharply as Sertoli cell proliferation ceases and spermatogenic differentiation begins. We hypothesized that changing activin levels also affect germ cell maturation. We measured an acute and developmentally-regulated impact of activin on Kit mRNA in co-cultures of Sertoli and germ cells from Day 8, but not Day 4, mice. Both stereological and FACS analyses identified an elevated spermatogonium:Sertoli cell ratio in Day 7 testes from Inhba(BK/BK) mice which have decreased bioactive activin, and germ cell markers Sycp3, Dazl, and Ccnd3, were significantly elevated in Inhba(BK/BK) mice. FACS measurements demonstrated surface KIT protein is significantly higher in Day 7 Inhba(BK/BK) germ cells compared to those from wild-type littermates. By Day 14, the germ:Sertoli cell ratio was not different between genotypes, but the transition of Type A spermatogonia into spermatocytes was altered in Inhba(BK/BK) testes. We conclude that regulated activin signaling controls not only Sertoli cell proliferation, as previously described, but also influences the in vivo progression of germ cell maturation in the juvenile testis at the onset of spermatogenesis.