Aromatase was localized in the porcine placenta by immunohistochemistry during the first peak of maternal estrogen concentrations (D25; n = 3), their nadir around midgestation (D50; n = 3) and their second increase in late gestation (D110; n = 3). On D25, aromatase was highly expressed throughout the trophoblast. On D50, its expression was restricted to the trophoblast lining the trough-like fossal regions between the bases of chorionic folds. However, immunostaining was detected only in a minor proportion of these locations, with staining intensity being generally only weak to moderate. On D110, distinct to intense immunostaining was found in virtually all trophoblast fossal regions and in similar structures at the flanks of chorionic folds adjacent to the free margins of secondary and tertiary endometrial folds. A weak staining of questionable specificity was occasionally observed in endometrial glands. In other cell types of the utero-placental compartment, aromatase was undetectable. The results suggest that in pigs placental estrogens could be involved in the formation of a more complex geometry of the feto-maternal interface as pregnancy progresses.