The endometrium of late pregnant guinea-pigs was found to contain mRNA for relaxin. Poly(A)+RNA hybridized to 3 2P-labelled porcine relaxin-specific oligonucleotide probes. These probes corresponded to the C-peptide region of the prohormone. There was no hybridization to a 3 2P-labelled probe to the B-chain of porcine relaxin even under conditions of low stringency. The size of the relaxin mRNA was approximately 1.0 kb and similar to that found for relaxin mRNA from the pregnant sow ovary. This is the first study on relaxin mRNA from a uterine source.