Previous studies have shown that iodide is actively taken up into mammary alveolar epithelial cells and secreted into milk. In the present studies we demonstrate that 125I also accumulates in MCF-7 cells against a concentration gradient; distribution ratios of greater than 30 were achieved. Iodide uptake into MCF-7 cells is transient, with peak accumulations occurring in about 5 min. The iodide is rapidly metabolized, probably to iodine, and it then exits the cells. The iodide transporter identified in MCF-7 cells is pendrin. DIDS, a nonspecific inhibitor of anion exchange, inhibits iodide uptake. Iodide uptake is impaired at reduced temperature, but is not dependent on sodium. Inhibitors of the sodium-iodide symporter (NIS) as well as ouabain did not affect the extent of iodide uptake. The pendrin transporter but not NIS was identified via western blotting techniques. Pendrin appears to be the primary iodide transporter in the MCF-7 cell line stocks that were employed for these studies.