Relaxin (RLX) mRNA was sought in the corpus luteum of the pig during the cycle, pregnancy, and lactation using Northern analysis and in situ hybridization. Three oligonucleotide probes to regions of the preprorelaxin molecule were used for hybridization to detect RLX message and gave similar results. Northern analysis showed a single 1-kilobase RLX transcript at all three stages of the reproductive cycle studied. The intensity of the RLX hybridization signal was greatest in pregnancy and varied during the cycle. The signal in ovaries from day 3 cyclic animals increased by day 13 and declined on day 19. However, the hybridization signal at midcycle was only 2% of that during pregnancy. After parturition on day 2 of lactation, RLX message was still detected in the ovary, although at reduced levels compared with those during the cycle and pregnancy. In situ hybridization results showed hybridization to RLX mRNA in luteal tissue on day 13 of the cycle, but not on day 3 or 19. An increased hybridization signal was observed on days 40, 60, and 90 of pregnancy, with a decline on day 2 of lactation. Control sections incubated with labeled heterologous probe or preincubated with excess unlabeled probe did not hybridize. These results indicate a good correlation between the relative concentrations of RLX transcript and immunohistochemical results previously reported in the corpus luteum of the sow. In addition, they demonstrate that the RLX gene is expressed in luteal tissue, not only in pregnancy, but also in the cycle and early lactation.