We have identified two highly conserved mouse genes encoding bone morphogenetic protein 8A (BMP8A/OP2) and 8B (BMP8B). The two loci are tightly linked on chromosome 4, suggesting that they arose through a recent gene duplication. Contrary to previous reports, neither gene is expressed in the early postimplantation mouse embryo (7.5-10.5 days post coitum) as judged by a variety of sensitive techniques. By contrast, high levels of Bmp8b RNA are found in the decidual cells of the uterus, and both genes are expressed in the trophoblast cells of the labyrinthine region of the placenta and in the inner root sheath of hair follicles of early postnatal skin. In addition, both Bmp8a and Bmp8b are expressed in the testis during specific stages of spermatogenesis, with the highest levels of RNA in stage 6-8 round spermatids after 3 weeks of age. Bmp8a and 8b are, therefore, the first members of the transforming growth factor beta (TGF beta)-related gene family to be found expressed in the germ cells of the testis, rather than in the somatic Sertoli cells. These results suggest that Bmp8a and 8b are not required for development of the embryo proper but regulate aspects of cell proliferation, survival and/or differentiation during spermatogenesis and placentation.