We investigated the concentrations of serum inhibin B and the pro-alphaC-containing inhibins (pro-alphaC inhibin) and their relations to serum FSH levels in 38 healthy boys during their progression through puberty. Furthermore, we studied the effect of recombinant human FSH (rhFSH) on pro-alphaC inhibin production in three prepubertal gonadotropin-deficient boys. The serum inhibin B level increased between Tanner stages G1 and G2 (p