The present studies indicated that the extensibility of the mouse uterine cervix was significantly increased by administration of as little as one unit of highly purified preparation of porcine relaxin. No significant change in tensile strength of the cervix was noted when doses were administered that caused increased cervical extensibility and formation of the interpubic ligament. A low coefficient of correlation was found between interpubic ligament formation and increased extensibility of the cervix in oestrogen-primed immature female mice.