Relaxin has been identified in term placentae of humans, and recently this hormone was identified in trophoblast cells of the implanting blastocyst and developing placenta of the hamster. The objectives of the proposed research are to characterize hamster placental relaxin and examine the secretion of relaxin using the hamster aa a model. Several experimental protocols will be used. In the first study, relaxin will be isolated from placentae and purified by chromatographic procedures. Molecular weight, isoelectric point and amino acid sequence of the purified product will be determine to allow comparison with relaxins from other species and tissues. In the second study antiserum to purified hamster relaxin will produced and an homologous hamster relaxin radioimmunoassay developed. This assay is critical for studying placental relaxin in this species. In the third study serum relaxin concentrations during gestation will be determined. In addition, the kinetics of placental relaxin synthesis and secretion (storage, rate, percentage of total protein synthesis, molecular weight of secreted hormone) will be determined for several stages of gestation. These variables will be examined by incubating placental explants with radioactive amino acid and measuring incorporation into total protein and relaxin. In study five the role of the pituitary and ovary in regulating placental relaxin synthesis and secretion will be examined by measuring serum and tissue hormone concentrations after removal of these organs after different stages of gestation. These studies will provide information on the biology of placental relaxin which may be useful in understanding its role in establishment and maintenance of pregnancy.