Whole saliva fluoride and DMF-S index were assessed in 13-year-old life-long residents in areas with 2.31 or 0.36 ppm F in the drinking water. Mean salivary fluoride concentrations in subjects from the high F area of 0.047 (morning) and 0.032 ppm (noon) were significantly higher than corresponding values of 0.026 and 0.017 ppm observed in the low F area. The mean DMF-S was 7.0 in the high and 15.2 in the low F area, thus being inversely related to whole saliva fluoride on a population basis. However, no such relationship could be demonstrated on an individual basis in either of the two areas. A 30-second mouthrinse with 10 ml of local tap water produced significantly increased mean fluoride levels in whole saliva attaining 0.092 and 0.032 ppm 15 min after rinsing in the high and low F areas, respectively. It was concluded that direct contact of the oral cavity with F in the drinking water is the most likely source of the elevated whole saliva fluoride and that the increased availability of fluoride in the oral fluids has an important relationship to the reduced caries progression observed in fluoridated areas.