Relaxin (RLN) belongs to a family of hormones structurally related to insulin and presents a broad spectrum of actions. Humans have three forms of RLN, encoded by three different genes (RLN1, RLN2 and RLN3), but nonprimate vertebrates have only two forms of relaxin (RLN1 and RLN3). RLN1 of these animals is encoded by Rln1, orthologous to the human RLN2 gene, and both genes, Rln1 and human RLN2, encode the major form of relaxin found in the male reproductive system. In the reproductive tract of human males, RLN is mainly produced by the prostate and secreted into the seminal fluid, where it seems to play a role in sperm function. RLN may also play a role in prostate cancer progression. A lack of RLN in animal models impairs male fertility, and RLN knockout mice display decreased sperm maturation. The precise role of RLN in the male reproductive system, however, is still far from clear. RLN action is due to its interaction with the G-protein coupled receptor RXFP1. Studies from our laboratory have shown that RLN and RXFP1 are expressed in rat Sertoli cells, and exogenous RLN stimulates Sertoli cell proliferation. RLN receptors can also be detected in rat germ cells at different stages of development, suggesting that RLN may play a direct role in spermatogenesis. The distribution of RLN/RXFP1, however, appears to be speciesdependent, because in the boar testis RLN production seems restricted to the Leydig cells, whereas RXFP1 is found in Leydig, Sertoli and germ cells. The coexpression of RLN and RXFP1 in several regions of the male reproductive system suggests that the peptide may act in an autocrine/paracrine fashion.