Chemical structure of inhibins is known: there are molecules constituted by two subunits alpha and beta (A or B). Inhibin displays actions at hypophyseal, hypothalamic and gonadal levels. Produced by granulosa cells in female and by Sertoli cells in male, inhibin synthesis is stimulated by FSH, androgens and reduced by hypophysectomy and progesterone. At the present time, there is no evidence for a signal from germinal cells to modify inhibin production. Inhibin secretion parallels with follicular maturation as aromatase activity whereas luteinization arrests its production. Sperm inhibin levels are correlated with spermatozoa number. Its administration to the sheep induces either an anovulation or an increase of ovulation rate according to the scheme of treatment.