A method combining radiotracer techniques with paper electrophoresis to investigate the optimal decomposition conditions for cobalt, copper, selenium, and zinc in rat liver samples is described. After oral and intraperitoneal administration of the respective nuclides of 60Co, 64Cu, 75Se, and 65Zn solutions, the livers of the tested rats are removed and decomposed with a nitric and sulfuric acid mixture. The completeness of decomposition is investigated by measuring the respective radionuclide-containing species in the decomposed samples by electrophoretic analysis. The results indicate that the Co- and Se-containing liver samples are more easily decomposed than those containing Cu and Zn under the specific conditions. The same decomposition effects are observed for ingested radioisotopes as for intraperitoneally injected ones. The possible connection of decomposition with the analytical error is discussed.