The secretion of follicle-stimulating hormone (FSH) by the anterior pituitary gland is regulated by the interaction of hypothalamic and gonadal hormones. Recently, proteins termed inhibins that selectively suppress FSH secretion have been purified and characterized from the gonadal fluids of several species. Antibodies to a synthetic peptide encompassing the amino terminal 25 residues of the recently characterized porcine inhibin were used to develop a specific radioimmunoassay (RIA) for inhibin and to neutralize endogenous inhibin during the estrous cycle of the rat. The administration of 20 international units of pregnant mare serum gonadotropin (PMSG) stimulated the secretion of inhibin in intact immature female rats, whereas ovariectomy caused an abrupt decrease in plasma inhibin concentrations that were not prevented by the injection of PMSG. The infusion of a polyclonal antiserum to inhibin, from 12 noon on proestrus to 1 a.m. on the morning of estrus, as well as its acute intravenous injection during diestrus I or II, caused an increase in plasma FSH (but not luteinizing hormone) concentrations. These results support the hypothesis of a feedback loop between the release of ovarian inhibin and FSH in the female rat.