This study describes the testicular levels of inhibin/activin subunits by Northern analysis and in situ hybridization and serum and testicular levels of inhibins A and B and activin A by enzyme linked immunosorbent assays (ELISA) during postnatal development in the rat. We show that serum inhibin A levels are less than 4 pg/ml throughout postnatal life. Serum inhibin B levels peak at 572 ± 119 pg/ml (mean ± se) at d 40 post partum (pp) before falling to 182 ± 35 pg/ml in mature males. Serum activin A decreases from 294 ± 29 pg/ml at d 6 to 132 ± 27 pg/ml at maturity. Within the testis, inhibin A levels fall from 0.330 ± 0.108 ng/g at d 15 to less than 0.004 ng/g at maturity. Inhibin B levels peak at 43.9 ± 4.2 ng/g at d 6 before falling to 1.6 ± 0.13 ng/g at maturity. Testicular activin A levels fall from 18.6 ± 2.2 ng/g at d 6 to 0.094 ± 0.013 ng/g at maturity. Northern profiles of testicular inhibin/activin subunits correlate with immunoreactive levels demonstrated by ELISA. In situ hybridization suggests that βA and βB subunit expression is largely restricted to the seminiferous tubule, particularly Sertoli cells, spermatogonia, and primary spermatocytes. These data support the view that inhibin B is the major inhibin in the male rat and that levels relate to Sertoli cell number and activity. Furthermore, the demonstration of high local concentrations of activin A during the period of Sertoli cell proliferation and the onset of spermatogenesis support its proposed role because a modulator of testicular development and function.