Previous studies from our group suggest that the synchronised onset of oestradiol secretion by preimplantation conceptuses from Meishan pigs may contribute to the enhanced prenatal survival characteristic of this breed (Ashworth and Pickard, 1998). We have recently reported that in vitro, the onset of oestradiol secretion by conceptuses can be regulated in a dose-dependent manner by an aromatase inhibitor (letrozole; Ashworth et al., 2005). Before commencing in vivo trials to assess the efficacy of letrozole to promote prenatal survival, it is important to determine whether exogenous letrozole reaches the uterine lumen and to establish the relationship between the dose administered and the impact on oestradiol secretion at the conceptus-uterine interface.