Meishan gilts in late pregnancy and after hysterectomy were used to investigate the production and secretion of progesterone, relaxin, PRL and GH at different reproductive stages. Relaxin levels peaked higher in pregnant (66 ng/ml) compared with hysterectomized (34 ng/ml) gilts. Progesterone levels decreased to 16 ng/ml on day 112 in hysterectomized pigs and remained there until day 120, while progesterone levels declined precipitously from day 112 and reached basal levels a day after parturition. Prolactin concentrations in hysterectomized pigs remained low, while PRL steadily increased reaching peak levels (39 ng/ml) at parturition and remained high during lactation. Growth hormone concentrations were similar in hysterectomized and pregnant pigs until day 114, thereafter lactating pigs had higher (P < 0.05) GH levels. The results show that PRL and GH secretion is regulated differently in hysterectomized gilts compared with pregnant gilts. Furthermore, the results confirm that progesterone and relaxin secretion in hysterectomized and pregnant gilts is similar at comparable reproductive stages;Relaxin antibody against porcine relaxin (antipRLX540) was produced in sheep and used to investigate whether it would impair parturition in late pregnant pigs. AntipRLX540 or PBS vehicle was administered intraperitoneally daily at 6-h intervals to pregnant Yorkshire gilts from days 103-120. Duration of delivery from first to last piglet was greatly delayed in antipRLX540 gilts compared with PBS controls (P < 0.05). The number of stillbirths was greater in antipRLX540--compared with PBS-treated controls (P < 0.05). The concentration of unbound or free relaxin was decreased (P < 0.01) compared with unextracted plasma of antipRLX540--treated on days 104-113. The present study indicates that in the pig elevated endogenous relaxin concentrations are essential for unimpaired litter delivery and postpartum survival rates of piglets;RU 486 alone or combined with relaxin was used to induce parturition in late pregnant first-calf heifers. Heifers were assigned randomly to three treatments; group 1 received RU 486 alone; group 2 received RU 486 plus relaxin; and group 3 received placebo treatment. Parturition occurred 43 h after treatment in group 1, 52 h in group 2 and 182 h in group 3, (P < 0.01). Calf birth weights were lower in hormone-treated heifers compared with control animals (P < 0.025). Heifers in groups 1 and 2 experienced less incidence of dystocia, however, placenta retention longer than 24 h was observed in hormone-treated heifers and none occurred in the control group. The results show that RU 486 alone or combined with relaxin induces delivery in late pregnant heifers and promotes calving ease without the detrimental effects of dystocia which may be a serious problem in first-calf heifers.