The organ distributions of four different Tc-99m dimercaptosuccinate complexes ("Tc-DMS", Complexes 1 to 4) were determined using mice and were evaluated as renal imaging agents. The highest kidney uptake was observed with Complex 2:21.7% of dose, 3 hr after injection. The biologic distributions and gel chromatographic analyses using carrier Tc-99 and Sn-113 indicate that there is little possibility of mixed metal complexes of the type Tc-Sn-DMS: rather they contain only Tc + DMS. The labeling procedure for Tc-99m DMS as a renal agent proceeds in two steps: a rapid formation of Complex 1, and a slower, rate-determining step from Complex 1 to Complex 2. A reproducible lyophilized kit has been prepared. The yield of complex 2 greatly depends on the reconstitution volume of 99mTcO4-; yield averages 89% using 2 ml of 99mTcO4-eluate.