Positron emission tomography (PET) imaging reporter genes, such as HSV1-tk and D(2) receptor genes, make it possible to visualise gene expression non-invasively and repetitively in vivo. However, these systems require the synthesis of complicated substrates and the availability of expensive PET equipment. Expression of the sodium/iodide symporter ( NIS) gene can be easily monitored with radioiodines and technetium-99m using a gamma camera. To evaluate the possibility of using NIS as an imaging reporter gene, we compared its characteristics with those of the conventional HSV1-tk gene. The CM cell line was made by transfecting the HSV1-tk gene into CT-26 (mouse colon carcinoma cell line). The CTN and CMN cell lines were then made by transfecting the NIS gene into CT-26 and CM. We measured the uptake of iodine-125 iodovinyldeoxyuridine ([(125)I]IVDU) and (125)I to evaluate the expression of the HSV1-tk and NIS genes, respectively. Each cell line was injected into four flank sites in Balb/c mice. The biodistribution study was performed after intravenously injecting [(125)I]IVDU and (131)I, and (131)I scintigraphy was performed for the evaluation of NIS expression. In vitro studies indicated that CTN and CMN had 40- to 79-fold and 150- to 256-fold higher uptake of (125)I than CT-26 and CM, respectively. Furthermore, CM and CMN showed 57- to 69-fold higher uptake of [(125)I]IVDU than CT-26 and CTN. NIS gene expression and (125)I accumulation were found to be directly correlated ( R(2)=0.923), as were HSV1-tk gene expression and [(125)I]IVDU accumulation ( R(2)=0.956). Calculated signal per unit NIS and HSV1-tk mRNA expression was 23,240+/-3,755 cpm and 34,039+/-5,346 cpm, respectively. In vivo study indicated that CTN and CMN had 2.3- and 5.8-fold higher uptake of (131)I than CT-26 and CM, and 1.8- and 3.5-fold higher uptake of [(125)I]IVDU than CT-26 and CTN. Scintigraphy using (131)I easily visualised CTN and CMN tumours. In conclusion, the NIS gene may be viewed as an imaging reporter gene with comparable performance to the HSV1-tk gene for monitoring target gene expression.