Relaxin and progesterone are produced in the corpora lutea of the pig. In hysterectomized gilts, insulin-like growth factor I, luteinizing hormone; and prolactin alone or in combination increase progesterone production by aging luteal tissue from day 110 post-estrus, but they are ineffective at days 113 and 116. In contrast, the same hormone treatments increase relaxin production by these luteal tissues from days 110 and 113 compared with those tissues from the animals on day 116. Insulin-like growth factor I augments the ability of luteinizing hormone and prolactin to increase progesterone and relaxin production by aging corpora lutea;The primary physiological actions of relaxin are to prepare the birth canal for the accommodation of developing the conceptus(es) and to facilitate delivery of the fetus(es). The growth and modification of the uterine cervix are important events during pregnancy. In the presence of estrogen, relaxin accelerates the growth and development of the uterus and cervix of prepubertal gilts. A treatment of six relaxin injections (every other day with total 1 mg relaxin) induces a 4- to 5-fold increase in the wet weight, dry weight of the uterus and cervix compared with that in vehicle-treated animals. The contents, but not the concentrations, of DNA, protein, and hydroxyproline (collagen index) of the uterine cervix are significantly increased by relaxin plus estrogen treatments; this implies that growth and development of the uterine cervix can be accelerated by an appropriate hormone combination of relaxin and estrogen;In a time- and dose-dependent manner, estrogen increases the number of relaxin binding sites on the cultured porcine uterine cervical cells. The Scatchard plot exhibits downward curvilinearity which implies that there are two classes of relaxin binding sites on these cells. The secretion of hydroxyproline from these cervical cells is synergistically stimulated by relaxin and estrogen, and this stimulation is blocked by co-incubation with actinomycin D or cycloheximide. The incorporation of [superscript]3H-proline into the collagen of cervical cells is enhanced by relaxin and progesterone, but RU 486 counteracts the stimulatory action of progesterone.