SummaryTo evaluate the role of gonadotropins in the control of Miillerian Inhibiting Substance (MIS) secretion, pregnant rats were injected with rabbit antiserum against luteinizing hormone releasing hormone (LHRH), and their pups replaced with follicle stimulating hormone (FSH) and human chorionic gonadotropin (hCG). The LHRH antiserum (LHRH-AS) was given at 13 and 20 days of gestation. Control dams were injected with an equal volume of normal rabbit serum. The male pups from mothers treated with LHRH antiserum were given 5 daily s.c. injections of the FSH, hCG, or vehicle. The male pups from mothers treated with normal rabbit serum were given vehicle s.c. Testicular fragments of 6 day old pups born to mothers treated with LHRH antiserum during pregnancy showed an increase relative to controls in MIS activity in a graded organ culture bioassay system (grade 3.4 + 0.3 vs. 2.3 + 0.2) ( P < 0.01). FSH given to pups from mothers treated with LHRH antiserum reduced testicular MIS secretion compared to vehicle treated pups from the same mothers (grade 2.3 + 0.2 vs.3.4 f 0.3) ( P c 0.01). Thus, postnatal injections of FSH after immunologic blockade of gonadotropins in utero reduced MIS activity of the testes to the same level found in testes of 6 day control pups (grade 2.3 f 0.2 vs. 2.3 k 0.2). In contrast, MIS activity remained high despite postnatal hCG injection in pups born to mothers given LHRH-AS (grade 3.4 f 0.4 vs. 3.4 f 0.3). These studies suggest that secretion of MIS is dependent on normal hypothalamic secretion of LHRH and may be inhibited by FSH. Speculation FSH inhibits MIS secretion.MIS and testosterone are the principal hormones which govern the embryonic development of male genitalia. MIS, first described in mammals by Jost (17, 18) causes regression during midgestation of the Mullerian ducts of the male rat (26,27). MIS is secreted by the testis from day 14 of fetal life to birth and, in progressively lesser concentrations, until day 21 of postnatal life. Testosterone induces development of the Wolffian duct into the vas deferens, seminal vesicle and epididymis (8); and its active metabolite, 5-dihydrotestosterone, stimulates the urogenital sinus, genital tubercle, genital folds, and genital swellings to form prostate, glans, penis, and scrotum (8).It is uncertain whether MIS is secreted autonomously or under extragonadal influences. Studies of the pituitary control of MIS have been inconclusive and conflicting. Maraud et aL, (23,24) demonstrated an increase in MIS activity of the 2-month-old chick testes after hypophysectomy at 1 month. However, GroenendijHuijbers and Burggraaff (12) noted a spontaneous return of MIS activity in the testes of the 4-month old chick. However, rats hypophysectomized at 20 -days of age. failed -subsequently .to demonstrate an elevation in testicular Mullerian inhibiting substance (1 1). Neither prolactin (Donahoe et al., unpublished data), nor placental fragments added to the in vitro culture influenced MIS activity.Previously, we demonstrated that an antiserum to lute...