Rat NIS (rNIS) genes were transfected into a human colon cancer cell line (SNU-C5) by lipofection. The transfected cells (SNU-C5N) exhibited an increase 125I uptake to a level 10 times higher than the untransfected SNU-C5 cells. The addition of 300 microM DIDS, an anion channel blocker, to the culture media led to a 2.35 times increase of 125I uptake in the cells. For the first 10 minutes, up to 70% of the cellular radioactivity was released into the medium. In the biodistribution study using SNU-C5N-xenografted mice, the %ID/g of the SNU-C5N tumors at 1, 3, 6, and 12 h after the 125I injection were 4.43%, 1.09%, 1.05%, and 0.05%, respectively, which were significantly higher than those for the SNU-C5 tumors (P<0.05). In tumor imaging, the SNU-C5N-xenografted tumor was clearly visible. In this study, NIS lipofection is efficient for triggering significant iodide uptake by a nonthyroidal tumor. However, for an increased therapeutic effect, the key issue is iodide retention in the target tissue.