The authors' findings evidence the possibility of the blood serum, liver, spleen, and bone marrow Fe, Cu, Mn, Co, Ni, and Zn measurements by the direct atomic absorption method without their separation and extraction. The technique is simple, meets the requirements to physicochemical analyses, and may be recommended for investigations in theoretical and practical medicine. The method is not devoid of a number of drawbacks, that limit the range of its applications; this concerns, among other things, Co measurements. The studies have demonstrated significant differences in the levels of trace elements in the tested biological material, which is explained by the species and organic characteristics of their metabolism.