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Relaxin has been assayed from a wide variety of vertebrates (Steinetz, Beach & Kroc, 1959; Hall, 1960) but its function is only known in a few eutherian mammals where it facilitates parturition through effects on the connective tissue of the genital tract and the pelvis. The highest concentrations of the hormone are found during the latter half of pregnancy in the ovaries, uteri and placentae. In some species, notably the rabbit (Marder & Money, 1944), relaxin also occurs in the peripheral blood in increasing amounts during pregnancy and falls abruptly after parturition. In marsupials, the possibility that relaxin is important at parturition in facilitating passage of the foetus through the pseudo-vaginal passage has been investigated (Tyndale-Biscoe, 1966) and a relaxin-like activity was detected in the corpus luteum but not the ovaries of the monovular possum, Trichosurus vulpecula, 11 days post-oestrus. In the possum, as in most poly-oestrous marsupials, gestation is shorter than one oestrous cycle (Pilton & Sharman, 1962) and no endocrine secretions additional to those present during the oestrous cycle are thought to be involved in the maintenance of gestation. Parturition occurs 17 days post-oestrus and, if relaxin is involved in the process, the amount of the hormone in the corpus luteum and in the blood might be expected to reach a maximum level at that stage of the 25-day oestrous cycle. In order to investigate this, fifteen adult female possums were trapped from the wild population around Canberra and kept isolated from males. Daily vaginal smears were taken until oestrus occurred, this being designated Day 0. Three animals were sampled on each of Days 0, 4, 9, 14 and 19 post-oestrus. Each animal was anaesthetized with nembutal, and heparinized. The ovaries were exposed by mid-ventral laparo¬ tomy and the Graafian follicle or active corpus luteum identified. The ovarian vein draining that ovary was cannulated and 12 to 13 ml blood collected during the ensuing half-hour. The left carotid was cannulated and the animal exsanguinated. The ovaries were then removed, the corpus luteum separated and both tissues weighed and frozen. The blood was separated and the plasma also stored frozen until assay. The urogenital tract was fixed in formol saline and, subsequently, sections were prepared from the region of the pseudovaginal passage.

RELAXIN ACTIVITY DURING THE OESTROUS CYCLE OF THE MARSUPIAL, TRICHOSURUS VULPECULA (KERR)