The gene encoding rhesus monkey relaxin has been investigated. A cDNA library was prepared using corpus luteal RNA from a pregnant rhesus monkey, cDNA clones encoding relaxin were isolated and the nucleotide sequence was determined. The amino acid sequence of rhesus monkey preprorelaxin, predicted from the cDNA, demonstrates that the sequence has not been strongly conserved when compared with that of man, although features characteristic of the relaxin molecule have been maintained. This structural information will allow production of rhesus monkey relaxin, leading to studies investigating the bioactivity of relaxin in a homologous primate system. Southern blot analysis indicated that there is only one relaxin gene in the rhesus monkey and baboon genomes. In this respect these primate genomes are different from the human genome which contains two relaxin genes.