Relaxin has been localized in corpora lutea (CL) of pregnant NMRI mice using the avidin-biotin complex immunocytochemical procedure and an antiserum against highly purified porcine relaxin. The immunostaining was measured by immunodensitometry. Relaxin immunostaining was first observed in luteal cells of type I gestational CL on day 11.5 (D11.5), For each investigated day, all CL were identically stained, and immunostaining was evenly dispersed all over the CL. Seventy-five percent of cells were stained at D11.5, and nearly all cells were stained betweeen D13.5 and D18.5. The staining intensity increased throughout the last half of pregnancy, reaching a maximum at Dig. A few hours before parturition, at D18.5, relaxin immunostaining decreased dramatically and reached the background level shortly after delivery. From our results we may conclude that, in murine CL, the number of relaxin-secreting cells and the intracellular storage of the peptide increase during pregnancy. The disappearance of relaxin with the cells occurs rapidly ± 12 h before parturition.