We have developed a sensitive and precise radioimmunoassay for porcine relaxin which we have employed for the determination of plasma relaxin levels throughout normal pregnancy in the pig. We intend to determine plasma relaxin concentrations in pregnant pigs under experimental conditions designed to provide insight into the factors which influence relaxin secretion. The radioimmunoassay developed with porcine relaxin is specific for pig relaxin and therefore rigorous studies requiring the determination of relaxin levels in the blood are presently confined to the pig. We plan to isolate rat relaxin and develop a sensitive radioimmunoassay for rat relaxin in order to extend the possibilities for physiological studies involving plasma relaxin levels to a common laboratory species. We propose to locate tissues which are responsive to relaxin in the female rat. We will initially administer low doses of biologically active, radiolabeled relaxin to ovariectomized adult female rats and determine the tissues which bind relaxin by autoradiography and direct counting of excised tissue. In vitro studies will then be conducted in order to determine whether the relaxin binding sites demonstrate the saturability, specificity, and affinity required to be defined as relaxin receptors.