Relaxin is abundant in porcine corpora lutea during pregnancy and after hysterectomy; the levels increase as the age of the corpus increases (Anderson, Ford, Melampy & Cox, 1973). It declines sharply before parturition and is associated with accumulation and disappearance of granules in the cytoplasm of the lutein cell (Belt, Anderson, Cavazos & Melampy, 1971). Cytoplasmic granules are abundant in rat corpora lutea during late pregnancy (Long, 1973). Holtzman-derived rats, weighing 150–180 g, were caged singly in an air-conditioned room with a lighting scheme of 12 h light and 12 h darkness. Oestrous cycles were determined daily by vaginal smears. Pseudopregnancy was induced by mechanical stimulation and the animals were killed on days 6, 9, 11 and 13. Hysterectomies were performed on day 6 of pseudopregnancy and the animals killed on days 6, 9, 11, 13, 17, 19 and 21. A decidual response was induced on day 4 and these