The overall objective is to determine the intraovarian and extraovarian endocrine relationships which govern ovarian function during pregnancy in the rabbit and rat. The mechanism of luteinization of Graafian follicles will be investigated using a procedure in which follicles are incubated in vitro and autotransplanted beneath the kidney capsule where they develop into ectopic corpora lutea. Follicles will be incubated with I125-hCG to determine the site of binding of labeled hormone and the role of hormone binding in subsequent luteinization. The contribution and functional significance of theca interna cells during luteinization will be studied by histologic and autoradiographic analysis of corpora lutea derived from follicles with "tagged" cells, and by transplantation of isolated follicular cells. The requirement for non-luteal ovarian tissues during pregnancy will be determined in relation to 1) survival of pregnancy, 2) maintenance of the corpus luteum and 3) maintenance of normal serum levels of progesterone, 17 beta-estradiol, 20 alpha-dihydroprogesterone, LH and prolactin which are measured by radioimmunoassay. Ectopic Graafian follicles will be established in pregnant animals to study their longevity, their ability to synthesize and secrete 17 beta-estradiol, their ability to luteinize in response to LH and their histological fate upon degeneration. Removal of small silastic capsules containing 17 beta-estradiol from intact pseudopregnant rabbits initiates rapid, premature regression of corpora lutea. This procedure will be used in pregnant rabbits to investigate the action of the presumed placental luteotropin in this species. Rates of structural and functional luteolysis will be compared in ectopic corpora lutea vs. in situ corpora lutea to determine the relative importance of local (intraovarian or uterine) factors vs. systemic factors in luteal regression. BIBLIOGRAPHIC REFERENCES: Keyes, P.L. and R.R. Houk. The postpartum gonadotropin surge in the rat depends upon the presence of nonluteal ovarian tissues. Biol. Reprod. 13: 45, 1975. Miller, J.B. and P.L. Keyes. Progesterone synthesis in developing rabbit corpora lutea in the absence of follicular estrogens. Endocrinology 97: 83, 1975.