Relaxin is a peptide hormone which has a variety of physiological effects on tissues of the reproductive tract as well as other organs such as the heart and brain. Whereas all non-primates so far examined have only a single relaxin gene, humans have two genes (H1 and H2, or gene 1 and gene 2). H2 relaxin is synthesized in the corpus luteum during pregnancy and is also found in the placenta and prostate, whereas expression of H1 has been very difficult to detect. We have begun a study of relaxin genes in the chimpanzee to assess whether this species may provide a suitable model in which to examine the roles of gene 1 relaxin. We find that the chimpanzee has two relaxin genes, one of which is very similar to H2. The second gene has an gene 1 type A chain but the B chain is of the gene 2 type, possibly due to a gene conversion event. The authentic chimpanzee gene 2 (Ch2) is expressed in the corpus luteum of pregnancy and in the placenta. Ch1 is not expressed in the placenta, but the mRNA can be detected by polymerase chain reaction in the corpus luteum.