Despite the documented importance of the protein hormone relaxin in reproduction in various mammalian species, the role of relaxin in human reproduction is poorly understood, largely because of the lack of studies in women or in suitable non-human primate models. Here we describe the establishment of a nonhuman primate model of early human pregnancy and its use in defining the actions of relaxin. Results demonstrate that relaxin exerts dramatic uterine effects including pronounced increase in uterine weight and stimulation of endometrial angiogenesis and resident endometrial lymphocyte number. In addition, relaxin decreases endometrial levels of matrix metalloproteinases 1 and 3 and increases levels of their endogenous inhibitor, tissue inhibitor of metalloproteinase 1, resulting in maintenance of endometrial collagen content. Relaxin significantly inhibits endometrial levels of estrogen receptor ␣, but not , and of progesterone receptor isoforms A and B. The findings that relaxin stimulates new blood vessel formation and increases cytokine-containing lymphocyte number while maintaining endometrial connective tissue integrity are consistent with a significant role of relaxin in the establishment and͞or maintenance of early pregnancy.R elaxin is a 6-kDa protein hormone member of the insulinlike growth factor family present in circulation in women during the latter part of the menstrual cycle and throughout pregnancy (1). In many mammalian species, relaxin exerts pronounced effects on the female reproductive tract that are involved in the maintenance of pregnancy and successful parturition (2, 3). Relaxin is important for normal delivery in several mammalian species because of its marked rearrangement of reproductive tract connective tissue (2-4). Despite the documented importance of relaxin in various mammalian species, the role of relaxin in human reproduction is, to date, an important, yet unanswered, question. Elucidation of the role of relaxin in women has been hampered by the inability to perform studies in women and by the lack of studies performed in suitable primate models of human pregnancy. Here we describe the establishment of a non-human primate model of early human pregnancy and its use in defining the actions of relaxin. Results demonstrate that relaxin exerts dramatic uterine effects including pronounced increase in uterine weight, stimulation of endometrial angiogenesis and resident lymphocyte number, maintenance of endometrial connective tissue integrity, and inhibition of endometrial estrogen and progesterone action. These effects are the developmental changes that occur in the human uterus during the late secretory phase of the menstrual cycle and early pregnancy. These findings support the thesis that relaxin acts as an important factor in uterine accommodation to and maintenance of early pregnancy in women.The dramatic species differences in the sources, secretion patterns, and target organs of relaxin have contributed greatly to the lack of understanding of the role of relaxin in human repr...