Members of the TGF beta superfamily may compete for receptor occupancy and intracellular signaling molecules in specific developmental circumstances. We explored the potential importance of the TGF beta family inhibitor, Bambi (Bmp and activin membrane-bound inhibitor) by examining its pattern of mRNA expression in juvenile and adult rat tissues, with a focus on reproductive organs. The 1.8-kb transcript was ubiquitous, whereas a 3-kb transcript was unique to enriched spermatocyte and spermatid cell fractions and adult testis. The full-length rat cDNA is 89% (nucleic acid) and 95% (amino acid) identical to its human homolog, hnma. Using in situ hybridization, Bambi mRNA was detected in granulosa and thecal cells of adult ovaries and in spermatogonia, spermatocytes, round spermatids, and Sertoli cells of adult testes. In addition to a persistent signal in Sertoli cells in juvenile testes, this mRNA within germ cells appeared dramatically increased as gonocytes matured into spermatogonia immediately after birth. These data indicate that TGF beta superfamily signaling within male germ cells is down-regulated at the onset of spermatogenesis. The addition of exogenous activin A to 24-h cultures of newborn rat testis fragments decreased the Bambi mRNA level. Regulated Bambi mRNA synthesis may contribute to TGF beta superfamily signaling modulation in several organs, as suggested by its discrete expression switch in male germ cells.