Although Na+/H+ exchange is important to maintenance of pH and volume and thus placental-fetal homeostasis, regulation of the Na+/H+ exchange system is incompletely understood. We previously showed that Na+/H+ exchanger (NHE)1 and -3 and their regulatory factors NHERF1 and -2 were expressed in human and nonhuman primate placenta. Our laboratories have also shown that estrogen regulates key aspects of primate placental function and development including the 11beta-hydroxysteroid dehydrogenase enzymes controlling cortisol metabolism. Therefore, it is possible that localization and/or expression of components of the syncytiotrophoblast NHE system are also estrogen dependent. As a first step in testing this possibility, the current study compared the immunocytochemical localization and level of NHE1, NHE3, NHERF1, and NHERF2 in baboon placentas obtained at mid- (d 100) and late gestation (d 165; term = d 184). NHE3 and NHERF2 were abundantly expressed at midgestation and localized to the cytoplasm and juxtanuclear compartment but were not detected in the microvillus membrane. By late gestation, NHE3 and NHERF2 expression were markedly reduced in the juxtanuclear compartment but not the cytoplasm. NHERF2 was also abundantly expressed in fetal vascular endothelium in which levels, as assessed by immunoblot exhibited a 3-fold developmental increase. In contrast, levels of NHE1 and NHERF1, which were abundantly expressed in and localized almost exclusively to syncytiotrophoblast microvillus membrane, were similar at mid- and late gestation. We conclude that the subcellular distribution and levels of key components of the Na+/H+ system in the baboon syncytiotrophoblast are developmentally regulated.