A 6-kD polypeptide, relaxin is classified as an insulin-like growth factor. Its actions include remodeling the connective tissues of the female reproductions tract in preparation for parturtion, modulation of sperm motility, and possibly a role in neurotransmission and atrail contracility. We study the secretion of relaxin, the mediation of its effects on connective tissues by metaoproteinases, its receptor-binding sites, and its possible role in pathological processes. The importance of relaxin in human physiology is presently unknown, although we have found that the pattern of serum relaxin observed in reproductive cycles of chimpanzees, but not marmosets, closely mimics that of women, suggesting the former are suitable primate models for human relaxin research. With collaborators at Cornell University College of Veterinary Medicine and New Jersey Medicine School, we investigate the roles of maternal estrogens and relaxin in the etiology of hip dyplasia (HD), an insidious precursor of osteoarthritis in later life. Labrador retrivers, genetically predisposed to HD, serve as human disease models. Microgram quantities of relaxin are secreted in colostrum and milk of dysplastic bitches, and following ingestion by the newborn, are readily absorbed into the circulation. The earliest detectable abnormally in pups predisposed to HD is a laxity of the hip joint capsule. Both laxity and HD are prevented by handrearing pups, thus avoiding consumption od colostrum and milk. We are now testing the hypothesis that anticanine relaxin antibodies and an inhibitor of estrogen synthesis will prevent joint laxity and subsequent HD in suckling, dysplasia-prone pups. A tangential but practical discovery also resulted from this work: Serum relaxin is a reliable indicator of pregnancy in the bitch.