The proposed research involves the synthesis and testing of a series of peptide analogs of luteinizing hormone-releasing hormone (LH-RH), designed to be antagonists of the native hormone. Design of the analog is based partly on the calculated conformations of LH-RH and some of its known antagonists and partly on the experience from previous work in this area. The peptides will be tested for their in vitro LH and FSH agonist and antagonist activity and for their anti-ovulatory activity in rats. The best antagonists will be tested for their contraceptive activity in rats and for their ability to prevent LH secretion in chimpanzees.