This study investigated the ontogeny of control of FSH secretion by inhibin during early prepubertal development of bulls by 1) measurements of circulating levels of inhibin and FSH from 1 to 13 wk of age, and 2) immunoneutralization of endogenous inhibin at 7, 21, 60, and 120 days of age. In addition, production and localization of inhibin in testes were examined by immunohistochemistry and Western blots at 7, 21, 60, and 120 days of age. Plasma immunoreactive inhibin levels were relatively low between 1 and 3 wk of age and then showed a tendency to rise (P < 0.1) from 4 wk of age. Circulating concentrations of FSH were low during 3 wk after birth and increased at 5 wk, remained high (P < 0.05) until 16 wk of age. Treatment with inhibin antiserum resulted in a significant (P < 0.05) increase in plasma FSH at 7, 21, 60, and 120 days of age compared to those following injection of control serum; however, the magnitude of the FSH rise after inhibin immunization was greater as bulls aged. There were no significant changes in plasma LH after inhibin immunization. An intense staining of inhibin α subunits was found in Sertoli cells within the solid seminiferous cords from 7 to 120 days of age, while no specific immune reaction was found in interstitial cells. Western blot analysis of testicular homogenates isolated from bulls 7–120 days of age revealed presence of a 28.5-kDa molecule that cross-reacted with inhibin α subunit and β_B subunit-specific antibodies. In this study, before 13 wk of age in bull calves, there was no inverse relationship between plasma concentrations of immunoreactive inhibin and FSH. However, the present immunization study clearly indicates that inhibin participates in the regulation of FSH secretion from infancy to early prepubertal stage, although the endocrine significance of inhibin becomes greater in older bulls. The results also indicate that the major production site of inhibin in the testis is Sertoli cells and that these cells produce inhibin that exerts a negative feedback effect on FSH secretion from early stages of development.