The objectives of two studies were to determine 1) whether plasma concentrations of inhibin (INH) changed with age in prepubertal bulls and whether these changes were related to changes in FSH, testosterone or testis length; and 2) whether castration and(or) estradiol implants affected plasma concentrations of INH and FSH. In Exp. 1, plasma INH remained constant from 4 until 8 wk of age then increased from 120 pM to 202 pM between 8 and 12 wk. Thereafter, INH decreased to 90 pM by 36 wk. Between 4 and 10 wk, plasma FSH increased from .32 to .43 ng/ml, apparently increasing before the initial rise in plasma INH. Between 10 and 12 wk, FSH declined from .43 to .33 ng/ml. After 12 wk, FSH increased as INH decreased. Initial increases in testis length and concentrations of plasma testosterone occurred at 14 wk coincident with the second rise in FSH. In Exp. 2, bull calves were either left intact, castrated, castrated and implanted with estradiol, or left intact and implanted with estradiol at 7.5 wk of age. Castration decreased concentrations of INH and increased concentrations of FSH. Castrated calves implanted with estradiol had decreased concentrations of both INH and FSH. Intact bulls implanted with estradiol had decreased concentrations of FSH relative to intact unimplanted bulls; however, concentrations of INH did not display the age-related changes observed in intact, unimplanted bulls. In summary, age-related changes in plasma INH and FSH occur in bulls. Furthermore, plasma concentrations of INH and FSH increased before changes in gonadal size were detected. The bovine testis may be a major source of circulating INH because castration decreased concentrations of plasma INH.