Organic anion transporting polypeptide (Oatp) 1c1 is a high-affinity T 4 transporter with narrow substrate specificity expressed at the blood-brain barrier. A transport model using cells overex- pressing Oatp1c1 was created to identify novel Oatp1c1 substrates and inhibitors. Rat Oatp1c1 was cloned and stably expressed in human embryonic kidney 293 cells. Oatp1c1-transfected human embryonic kidney 293 cells transported 125I-labeled T 4 in a time-dependent manner that was completely abolished in the presence of excess unlabeled T 4. Next, various compounds, including inhibitors of thyroid hormone uptake, were screened for inhibitory effects on Oatp1c1-mediated T 4 uptake. Phenytoin (64%), indocyanine green (17%), fenamic acid (68%), diclofenac (51%), and meclofenamic acid (33%) all reduced T 4 uptake by Oatp1c1 when assayed at concentrations of 10 μM. Dose-response assays for the fenamic acids, iopanoic acid, indocyanine green, and phenytoin revealed IC 50 values for Oatp1c1 T 4 uptake below or near the blood plasma levels after therapeutic doses. Further kinetic assays and reciprocal plot analyses demonstrated that the fenamic acid diclofenac inhibited in a competitive manner. Finally, microvessels were isolated from adult rat brain and assessed for T 4 uptake. Ten micromolar of fenamate concentrations inhibited T 4 mi- crovessel uptake with a similar hierarchical inhibition profile [fenamic acid (43%), diclofenac (78%), and meclofenamic acid (85%)], as observed for Oatp1c1 transfected cells. Oatp1c1 is expressed luminally and abluminally in the blood-brain barrier endothelial cell, and exhibits bidirectional transport capabilities. Together, these data suggest that Oatp1c1 transports fenamates into, and perhaps across, brain barrier cells.