It is currently believed that the fertility level of the adult mammalian testis is related to the total number of Sertoli cells, which is established in the early prepubertal life. We have previously reported that, in an in-vitro system, terminal Sertoli cell proliferation is sustained by activin A in concert with FSH. In this paper, we have addressed the question of whether this activin A effect correlates with activin receptor II (ActRII) expression pattern during early post-natal testis development. We first determined the precise developmental interval of activin proliferative effect on Sertoli cells in vitro and then analysed the expression of ActRII in purified testicular cell populations by Northern blot and in-situ hybridization. While the 3 kb ActRII isoform was widely expressed at different ages and in several testicular cells, including Sertoli cells, germ cells and myoid cells, the canonical 6 kb ActRII isoform was specifically and transiently expressed at a high rate in Sertoli cells at 7-9 days after birth, the time when these cells respond to activin A in vitro. In the light of these results, we conclude that activin A regulates terminal Sertoli cell proliferation in the rat testis and that this effect is mediated by the 6 kb isoform of ActRII.