Pregnant Standardbred mares were allocated to 2 groups. On Day 45 of gestation, 20-45 ml saline (240 g NaCl/l) were injected into the fetal sacs of 10 mares, and the other 10 mares were given sham treatment. Post-operative plasma oestrone sulphate concentrations were lower (P less than 0.01) on Days 48-55 in saline-treated mares than in sham-treated mares. Mean plasma progesterone profiles were similar in the two groups of mares, although post-operative luteolysis occurred in 4 saline-treated mares. There was no difference in plasma CG profiles between the 2 groups, except that CG concentrations in saline-treated mares were generally lower than those of sham-treated mares. There was generally a post-operative loss of uterine and cervical tone in saline-treated mares. These results show that the maintenance of maternal plasma oestrone sulphate concentrations requires the presence of a viable embryo and confirm that luteolysis can occur despite high plasma CG concentrations after fetal death.