Relaxin is a peptide hormone produced by the corpora lutea of ovaries during pregnancy, softening and lengthening the ligaments of the pelvis and softening the cervix in order to make childbirth easier. In attempts to determine the nucleotide sequence coding for relaxin, recombinant DNA techniques were used to obtain a cDNA clone bank from total mRNA isolated from the ovaries of pigs in late pregnancy. Clones were screened using cDNA initiated by synthetic oligonucleotide primers coding for the Trp Val Glu Ile sequence of the porcine relaxin B chain. The synthetic undecamer [5'-ATCTCCACCCA-3'] was found to prime a specific 32P-labeled cDNA of approximately 300 nucleotides containing B chain and signal peptide coding sequences, as verified by nucleic acid sequence analysis. This cDNA was used to probe the ovarian clone bank. Several clones containing large inserts which hybridized to this probe were subjected to sequence analysis and some of these were found to contain the preprorelaxin coding region, comprising a signal peptide of 24 amino acids, a B chain of 32 amino acids, a large C peptide of 104 amino acids, and an A chain of 22 amino acids. From the amino acid sequence of prorelaxin derived in this way, it appears that the processing of prorelaxin involves two enzymes with chymotrypsin-like and trypsin-like specificity, respectively. In comparisons of porcine and rat preprorelaxins, the C region had as much amino acid sequence homology as the B and A chains. The C region is also rich in charged amino acids, suggesting a role for it beyond simply ensuring proper disulfide bond formation.