The heavy metals include at least 40 elements but cadmium, lead, and mercury have been most extensively studied. The biological properties of heavy metals are discussed in terms of three important characteristics: the ability to form, irreversibly, complexes and chelates with organic ligands; the properties to form organic-metallic bonds; and the potential to undergo oxidation-reduction reactions. The formation of complexes and chelates within the body is shown to influence greatly the dynamics of transport, distribution, and excretion of several important metal cations. The excretion of uranium is influenced by acid-base balance in the body because uranium forms complexes with bicarbonate anions that are filtered by the kidneys. The biliary excretion of methylmercury depends on the formation of small molecular weight complexes with sulfur-containing amiono acids and the peptides in the liver. The degree of enterohepatic recirculation of a variety of heavy metals appears to depend on the chemical nature of the bilary complexes. The oxidation of elemental to divalent ionic mercury is the crurial step in the retention and tissue deposition of inhaled mercury vapor. That the oxidation process is, at least in part, catalyzed by the enzyme, catalase, explains the effects of ethanol, aminotriazole and the state of acatalasemia on the metabolism of inhaled vapor in man and animals. The formation of covalent bonds between metal cations and the carbon atom usually greatly modifies the biological properties of the metal. Methylarsenic and methylmercury compounds both differ from the inorganic forms in accumulation in animals.