This review focuses on the role of the inhibin-related peptides, inhibin, activin, follistatin and α-N, in the hormonal regulation of reproduction, using data from the sheep, cow, pig and rat. Emphasis is placed on recent advances and remaining gaps in knowledge of the roles of these substances. The evidence confirms a role for inhibin as a circulating hormone, regulating the secretion of FSH in concert with oestradiol. Although follistatin is found in peripheral blood, its role as a circulating hormone remains uncertain; more likely follistatin acts as a binding protein for activin in its actions as a local regulator in many tissues including the ovary. α-N, a post translational product of the α chain of inhibin, appears to have a role in the control of ovulation. A major challenge is to develop more specific assays for all these peptides which will allow discrimination between the secreted and circulating forms.