Abstract: We use NaI(Tl) detector and multichannel analyser to record the net integral counts of ~(40)K characteristic γ-rays both in living small animal and its imitated phantom. This method may analyse quantitatively the potassium content in living small animals. Comparing the results of radioactive Assay and Atomic Absorption for 20 mice weighing 200—325 g, it was found that their deviation was less than 12%.