Transporters within the placenta play a crucial role in the distribution of nutrients and xenobiotics across the maternal-fetal interface. An organic anion transport system was identified on the apical membrane of the rat placenta cell line HRP-1, a model for the placenta barrier. The apical uptake of [³H]-labeled organic anion estrone sulfate in HRP-1 cells was saturable (Km=4.67 µM), temperature-, Na⁺-dependent, Li⁺-tolerant, and pH-sensitive. The substrate specificity of the transport system includes various steroid sulfates such as -estradiol-3,17- disulfate, 17-estradiol-3-sulfate, and dehydroepiandrosterone-3-sulfate (DHEAS), but does not include taurocholate, para-aminohippuric acid (PAH), and tetraethylammonium (TEA). Preincubation of HRP-1 cells with 8-bromo-cAMP (a cAMP analogue) and forskolin (an adenylyl cyclase activator) acutely stimulated the apical transport activity. This stimulation was further enhanced in the presence of 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor). Together these data show that the apical membrane of HRP-1 cells expresses an organic anion transport system, which is regulated by cellular cAMP levels. This transport system appears to be different from the known taurocholate-transporting organic anion transporting polypeptides (Oatp) and PAH-transporting organic anion transporters (OAT), both of which also mediate the transport of estrone sulfate and DHEAS.