Inhibin is a dimeric glycoprotein that suppresses follicle-stimulating hormone (FSH) secretion from the pituitary. Two bioactive forms of inhibin exist, inhibin A and B. The availability of specific immunoassays for each of the isoforms has enabled the study of the individual inhibins and their physiological roles. In the male, inhibin B is the circulating form in all species studied to date except the sheep. Inhibin B is produced in the testis, principally by the Sertoli cells. There are temporal changes in inhibin expression and secretion with the changing role of the Sertoli cell in immature and adult testes. Variations in inhibin B production between species reflect the different patterns of maturation. In the adult, inhibin B levels are positively correlated with Sertoli cell function, sperm number, and spermatogenic status and are negatively correlated with FSH. It appears that the regulation of inhibin B production is mediated by a complex interaction between FSH, Sertoli cells, Leydig cells, and germ cells. Inhibin may also play a role at an autocrine or paracrine level in modulating the actions of activin.