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The role of fluorine in promoting resistance of teeth to decay has been supported by evidence from three sources: (a) chemical analyses for fluorine in teeth, (b) studies of the incidence of caries in the human as correlated with the fluorine content of drinking water and milk (1) and (c) animal experimentations. The literature referring to the first two lines of evidence has, for the present purposes, been adequately summarized in other places (2, 3). Miller (4) reported the rate of appearance of caries in the rat to be decreased by the addition of large amounts of sodium fluoride or sodium iodoacetate to the food. This observation was confirmed by Hodge and Finn (5) who found the daily oral administration of 3 mg. fluorine in solution to rats reduced, not only the incidence of caries, but also the occurrence of fracture of the cusps of the molar teeth. Cox, Matuschak, Dixon, Dodds and Walker (6) reported that the addition of 41.2 p.p.m. fluorine to the diet of rats during pregnancy and lactation brought about an increased resistance of the teeth of the offspring to caries. Cheyne (7) removed the salivary glands of rats and found that fluorine still exerted a protective effect against caries from which observation he concluded that the influence of fluorine is,not mediated through the secreted oral fluids. McClure and Arnold (8) observed that water containing 2 p.p.m. fluorine had no effect on the incidence of caries in the rat. However, they noted that 125 p.p.m. fluorine in the form of sodium fluoride in either food or water definitely reduced the caries attack rate in their experimental animals. Arnold and McClure (9, 10) recorded observations which indicated that sodium fluoride administered by subcutaneous injection produced an elevation of the fluorine content of the enamel of the molar teeth of young rats without increasing the caries resistance of the same teeth. They also observed that water containing 10 p.p.m. fluorine furnished only slight, if any, protection against caries in the rat. Sognnaes (11) found a probable reduction of the caries attack rate when concentrated solutions of potassium fluoride were applied to the surface of the molar teeth of rats. McClendon and Foster (12) reported a delay of occurrence of caries in the rat when drinking water containing 20 p.p.m. fluorine was given to rats receiving a diet containing only 0.3 p.p.m. fluorine.