Chinese Meishan and Yorkshire were used to investigate mechanisms regulating the production and secretion of relaxin, progesterone, prolactin (PRL), and growth hormone (GH) during late pregnancy and lactation. Uterine surgical removal (hysterectomy) of nongravid gilts during the estrous cycle (day 8) extends luteal function to 150 days which is beyond the duration of normal pregnancy. Hysterectomy provides a useful model to examine shifts in hormone secretion at the time of expected parturition in gravid pigs. Blood samples were collected via an indwelling jugular cannula twice daily (0800 and 2000 hours) from days 90 to 120 and every 20 minutes within a 3-hour period on days 112 to 116. Relaxin and progesterone are hormones of ovarian origin, and PRL and GH are hormones secreted by the pituitary gland. Parturition occurred earlier (day 114) in Yorkshire than Meishan (day 115) gilts. The prepartum peak relaxin release occurred about 16 hours earlier in both breeds, thereafter relaxin dropped to basal levels during lactation. After hysterectomy, relaxin plasma levels were consistently greater in Meishan compared with Yorkshire gilts from days 110-118. A programmed peak relaxin release occurred one day earlier in Yorkshire compared with Meishan gilts. Following the relaxin peak, corpora lutea persisted in both breeds, but Meishan gilts continued to secrete consistently greater amounts of relaxin than Yorkshire gilts. Progesterone plasma levels remained higher longer in late pregnant Meishan compared with Yorkshire gilts. After hysterectomy, progesterone plasma levels were consistently higher in Meishan compared with Yorkshire gilts from days 101 to 118. Prolactin circulating concentration increased during late pregnancy and early lactation, but at a higher level in Yorkshire compared with Meishan gilts. Growth hormone blood levels increased only during late pregnancy and early lactation in both Meishan and Yorkshire gilts. These results indicate significant differences in the timing, and in some cases, magnitude of hormone secretion profiles in pregnant and hysterectomized Yorkshire and Meishan gilts.