Relaxin is a peptide hormone whose A- and B-chains are derived by posttranslational cleavage from a single 185-amino acid preprorelaxin. Two genes in the human genome (H1 and H2) code for two polypeptides significantly different in amino acid sequence. The full spectrum of biological activities of these two polypeptides has not been examined, but transcription appears to be limited to the H2 relaxin gene in the human corpus luteum. Relaxin is also synthesized by the human decidua, placental trophoblast, and prostate gland; therefore, the expression of the human relaxin genes in these tissues has been examined using the reverse transcription polymerase chain reaction. The mRNA from decidua, placental trophoblast, and prostate was reverse transcribed and then amplified by polymerase chain reaction, using a series of oligonucleotide primers that were specific for but would not distinguish between human H1 and H2 relaxins. Using mRNA from these tissues, two amplified cDNA species were detected, whose identities were confirmed by Southern blots, HpaI and HpaII restriction enzyme analysis, and dideoxy sequencing. We have confirmed that the corpus luteum does not contain detectable H1 relaxin mRNA. However, we demonstrated for the first time relaxin H1 gene expression in the decidua, placental trophoblast, and prostate, and we have also shown that there are marked tissue differences in the relative amounts of expression of the H1 and H2 relaxin mRNA forms. The functional significance is unknown, but if both mRNAs are translated, differential expression of the two genes may result in tissue-specific differences in the production of these relaxins as well as in their binding and actions.