Recently, a novel Slc22 gene family member expressed in murine olfactory mucosa was identified and based on sequence homology proposed to be an organic anion transporter; Oat6 (Slc22a20) (31). However, no functional data for Oat6 was reported. In the present study, we demonstrate that murine Oat6 mediates the inhibitable transport of estrone sulfate using both Xenopus oocyte expression assay and Chinese hamster ovary (CHO) cells stably transfected with mOat6 (CHO-mOat6). Uptake was virtually eliminated by probenecid and the anionic herbicide 2,4-dichlorophenoxyacetate. The organic anions ochratoxin A, salicylate, penicillin G, p-aminohippurate, and urate inhibited mOat6-mediated accumulation to varying degrees. Transport of estrone sulfate by mOat6 was demonstrated to be saturable and K_m estimates of 109.8 ± 22.6 µM in oocytes and 44.8 ± 7.3 µM in CHO-mOat6 cells were obtained. Inhibitory constants (K_i) for 2,4-dichlorophenoxyacetate (15.7 ± 2.0 µM), salicylate (49.0 ± 4.4 µM), probenecid (8.3 ± 2.5 µM), and penicillin G (1,450 ± 480 µM) were also determined. Accumulation of estrone sulfate mediated by mOat6 was significantly trans-stimulated by glutarate indicating that mOat6 functions as an organic anion/dicarboxylate exchanger. These data demonstrate for the first time that the novel murine gene Oat6 (Slc22a20) encodes a functional organic anion transporter and mOat6 is indeed the newest member of the OAT gene family.