The effect of orally administered calcium and phosphorus on the fluoride balance was studied in man during a low fluoride intake of about 4 mg/day and a high fluoride intake of 14 mg/day. Three calcium intake levels averaging 223, 1,481, and 2,312 mg and two phosphorus intakes averaging 915 and 1,430 mg/day were used. Calcium or phosphorus used singly or combined did not affect the urinary fluoride. Increasing the calcium intake from an average of 223 to 1,481 and to 2,312 mg/day during the low fluoride intake increased the fecal fluoride only slightly, from 0.13 to 0.17 and 0.20 mg/day, respectively, and during high fluoride intake from 0.31 to 0.58 and 0.56 mg/day, respectively. The addition of phosphorus to the low calcium intake did not change the fecal fluoride excretion during either low or high fluoride intake, but the fecal fluoride increased during the two higher calcium intakes by a factor of 1.5. As the fecal fluoride was very low in the control study and was still relatively low in the experimental studies, the increase in fecal fluoride did not change the fluoride balances. The studies have shown that the increased levels of calcium and phosphorus used had little effect on fluoride metabloism in man.