The overall objective of the research is to elucidate the mechanism governing first ovulation in the sheep at puberty. Initially, we propose to determine if the sequence of endocrine events leading to first ovulation is (a) a decrease in sensitivity of the hypothalamo-hypophyseal system governing tonic LH secretion to the negative feedback action of estradiol, (b) an increase in circulating estradiol in response to the increase in circulating estradiol in response to the increase in circulating LH and (c) onset of the surge mode of gonadotropin secretion in response to the increase in circulating estradiol. We will determine if the rise in basal concentrations of LH immediately before first ovulation can be accounted for by mechanisms other than a decrease in sensitivity to inhibitory estradiol feedback. Changes in metabolic clearance rate and binding proteins for estradiol will be examined. Using a model system to be developed during the course of the research, the influence of environment, photoperiod, and body weight on the mechanism governing sensitivity to the negative feedback of estradiol during sexual maturation will be investigated. We will determine if the first LH surge at puberty (priming surge) and the transient rise in circulating progesterone (preluteal progesterone) which follows play a role in establishing normal corpus luteum function following the second LH surge. In this regard, it will be determined if preluteal progesterone influences the timing of the second LH surge and if the priming LH surge induces follicular changes such as distribution of gonadotropin binding. Several approaches and techniques will be used in the proposed research. They include controlled delivery of physiologic doses of steroid hormones, pulsatile infusion of LH, radioimmunoassay of circulating hormones, controlled photoperiod, body weight regulation by controlled food intake and localization of receptor sites in ovarian tissue.