We localized alkaline phosphatase and plasma membrane calcium-ATPase (PMCA) in the cat placental syncytiotrophoblast to address their polarized distribution and their potential as markers for specific plasma membrane purification. We used enzyme- (alkaline phosphatase) and immuno- (PMCA) histochemistry and, for alkaline phosphatase, compared data to observations on the human placenta. Alkaline phosphatase activity in the cat was localized to the decidual cell membranes, to within the associated interstitial space and on the subjacent apical (maternal facing) plasma membrane of the syncytiotrophoblast. Occasional maternal capillaries were positive on their basal surface and there was focal staining within the syncytiotrophoblast. This widespread distribution is less specific than in the human placenta where alkaline phosphatase was restricted to the apical and basal plasma syncytiotrophoblast membranes, with much greater density on the apical membrane. Expression of PMCA in the cat was restricted to the basal membrane of the syncytiotrophoblast only. This specific localization of PMCA is identical to the human placenta and all other species in which its placental localization has been studied. We conclude that the plasma membranes of the cat syncytiotrophoblast show a broadly similar functional polarization to the human and that PMCA would prove a useful marker in isolation of the cat syncytiotrophoblast basal plasma membrane.