Osborne-Mendel rats 22–25 days of age were subjected to a 7-day precaries test period during which they were fed a high starch diet and drank distilled water containing no added fluoride or with 10, 50, 100 or 150 ppm fluoride as sodium fluoride. After this period, 36 animals from each group were provided with distilled water during a 56-day caries test period (fed diet 2000 and infected with Streptococcus mutans 6715). Fluoride content in the enamel was shown to be directly related to the level of fluoride in the drinking water during the initial 7-day exposure period, both at the end of this 7-day period of fluoride treatment and at the end of the caries test period. Caries development was inversely related to fluoride uptake in the enamel; animals which received 50 ppm or more fluoride developed significantly fewer carious areas than the controls. 36 additional animals from each pretest group were subjected to the same caries test challenge as above but in addition were provided with 10 ppm fluoride in the drinking water. Caries development in these groups was extremely low even though there was no increase in the fluoride content of enamel in animals drinking water containing 10 ppm fluoride during the caries test period. Fluoride provided prior to the caries test regimen conferred the most protection on the sulci, in contrast, fluoride provided during the caries test regimen had a more pronounced effect on the smooth surfaces and in addition had a greater overall effect.

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