Carriers may mediate the permeation across enterocytes for drug substances being organic anions. Carrier mediated permeation for the organic anions estrone-3-sulfate (ES) and glipizide across Caco-2 cells were investigated kinetically, and interactions on involved carriers evaluated. Initial uptakes ( P UP) at apical and basolateral membranes, apparent permeabilities ( P APP) and corresponding intracellular end-point accumulations ( P EPA) of radioactive labeled compounds were studied. Possible effects of other anionic compounds were investigated. Apical P UP and absorptive P APP for ES were inhibited and its absorptive P EPA prevented in presence of the investigated organic anions and apical P UP was saturable with K m 23 μM. Basolateral P UP and exsorptive P APP were inhibited, its exsorptive P EPA was prevented, and basolateral P UP and exsorptive P APP were saturable with K m 44 μM and 38 μM, respectively. BCRP inhibition affected both absorptive an exsorptive P EPA and P APP for ES. Glipizide apical P UP and absorptive P APP were not inhibitable. Basolateral P UP for glipizide was inhibitable, its P EPA prevented, and P UP was saturable with K m 56 μM, but exsorptive P APP was not affected. Carrier mediated exsorption kinetics for ES are seen at both apical and basolateral membranes, resulting in predominant exsorption despite presence of absorptive carrier(s). Carrier mediated basolateral P UP for glipizide was observed, but glipizide P APP was not described by carrier kinetics. However, glipizide is affecting exsorption for ES, due to interactions on basolateral carrier. The study confirms that estrone-3-sulfate can be used to characterize anionic carrier kinetics. Furthermore it is suggested that estrone-3-sulfate may be used to identify compounds which may interact on anionic carriers.