Relaxin is a peptide hormone of luteal origin with a broad range of biological activities on tissues and organs of the female reproductive system as well as on other targets not directly related to the reproductive function. The mammary gland is one of the major targets for relaxin, which has been shown to promote growth and differentiation of mammary parenchyma and stroma. Based on the recognition of the mammotrophic action of relaxin, further research could show that this peptide also influences the behaviour of breast cancer cells in vitro. In fact, when relaxin was added to the culture medium of MCF-7 breast adenocarcinoma cells for short exposure times it had a biphasic effect on their growth, stimulating cell proliferation at low, nanomolar concentrations and inhibiting it at high, micromolar concentrations. In the longer times, relaxin had a marked growth-inhibitory effect on MCF-7 cells at any concentration assayed, and concurrently promoted cell differentiation and expression of adhesion molecules which are known to binder the spreading ability of cancer cells. The positive effect of relaxin on MCF-7 cell differentiation was even enhanced when these cells were cocultured with myoepithelial cells, thus recreating a microenvironment reminiscent of the tissue architecture of the mammary ducts in vivo. Concerning the mechanisms of action of relaxin on MCF-7 cells, it seems that the growth-inhibiting and differentiation-promoting effects of the peptide are mediated through the activation of the synthetic pathway of nitric oxide.