Despite the 1,000-fold difference between the fluoride concentrations ([F]) in dentifrices and fluoridated drinking water, clinical and epidemiological studies have shown that they have similar cariostatic effects. This double-blind, crossover study was done to determine whether the [F] in dental plaque is related more to the [F] of the dentifrice used or to the plaque concentrations of calcium and magnesium. The subjects (n = 13) were adults and residents of a city served with fluoridated drinking water. After 1 week of using a fluoridated dentifrice (940 ppm) or a placebo dentifrice, whole saliva and plaque were collected 1.0 h and approximately 12 h after the last use of the dentifrices. The average salivary [F] after brushing with the F dentifrice was higher than after using the placebo. The average plaque [F] 1.0 h after brushing with the F dentifrice was higher than after using the placebo (p < 0.01), but the difference at 12 h was not significant. Plaque [Ca] and [F] were directly related under all experimental conditions (p = 0.0001). The relationships between plaque [Mg] and [F] were weaker and inconsistent. Based on these findings and reports in the literature it is concluded that, for persons whose drinking water is fluoridated, plaque [F] throughout much of the day is not significantly increased by the use of a fluoridated dentifrice. Instead they are directly related to plaque [Ca]. These findings offer at least a partial explanation for why fluoridated dentifrices and drinking water have similar cariostatic effects.