Biological distribution of 103Ru in mice bearing Ehlrich's solid tumor and rats bearing AH-130 solid tumor were studied with two compounds of 103Ru, the chloride and citrate. The radioactivity of 103Ru-chloride showed slower excretion and higher tumor concentration than that of citrate. The tumor-to-muscle ratios of chloride were always higher, but the tumor-to-blood ratios were always lower than those of citrate. No significant difference was founds between 103Ru-chloride and citrate as tumor scanning agents.