Once absorbed, radionuclides enter the circulatory system and are distributed into various tissues of the body. In some cases, radionuclides are bio­transformed within tissues and may be present within the animal in more than one form. For instance, 3H occurs as tissue water or organically bound tritium incorporated into the protein and fat of tissues.

Different radionuclides are accumulated in different tissues. For some radionuclides, the site of deposition is determined by the biological role of the corresponding stable element or analogue. The major iodine storage organ in the body is the thyroid and the element is also actively taken up by the mammary gland and transferred into milk. Radiostrontium behaves as a calcium analogue and is therefore accumulated in bone and shell and is also transferred into milk. Radiocaesium is an analogue of potassium and is, therefore, found in all soft tissues. The actinides and rare earth elements are all accumulated in bone. Liver (or hepatopancreas in arthropods and gastropods) and, to a lesser extent, kidneys are common storage tissues for many pollutants including some radionuclides (e. g. actinide elements and heavy metals).