During the past quarter of a century there has been considerable investigation into the effects of fluorides on living organisms. It has been well established, as a result of these studies, that both small and elevated amounts of fluorides present in the environment may have a marked toxic effect upon gaining entry into the organism. Much of the research involving fluorides and the living organism has been confined to experimentation with animals, although considerable evidence indicates that plants are also subject to injury if fluorides are present in the atmosphere or the soil. The great majority of the research dealing with effects of fluorides on animals has been confined to higher vertebrates, principally domestic farm species. The concentration of fluoride research in this area has been prompted by economic pressures arising from natural, industrial, or accidental fluoridation of livestock. For the most part, these investigations have been confined to determination of toxicity levels to various domestic species, while the mode of fluoride action upon living organisms has often been neglected. Some investigators, however, have postulated "defense mechanisms" within the organism whereby active fluoride ion is removed from systemic circulation and is either deposited in less active tissues of the body, or, is excreted in some manner. Such mechanisms would offer a degree of temporary protection of the organism from harmful effects of the fluoride ion.