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To test the hypothesis that relaxin is an important factor supporting implantation, two approaches have been carried out using a human-relevant animal model, the marmoset monkey. First, uterine mRNA transcription and protein expression during the implantation phase in the conceptive and nonconceptive cycles were examined. Second, functional parameters were analyzed to assess the in vivo effects of exogenous applied relaxin throughout implantation. Relaxin and its receptor, RXFP1, were highly upregulated shortly before and during the physical process of implantation, indicating that relaxin is an important factor for remodeling and immunotolerance. The action of relaxin on the uterus was accompanied by an increase of estrogen-associated factors and macrophage infiltration, suggesting redundant systems necessary for successful implantation. The data from relaxin-treated animals supported those obtained from naive tissues in terms of increases in angiogenesis as well as earlier and faster growth of the uterus and placenta in the relaxin-treated marmoset monkey group, resulting in parturition 7-10 days earlier than the control group, but not pathological. In general, relaxin is very effective in preparing the endometrium for implantation. These findings should encourage further clinical research regarding introducing relaxin for pathological pregnancies, such as early pregnancy failure or insufficient placenta.