In a program of studies of disuse osteoporosis, fluoride balances were determined in healthy men during ambulation and then during bed rest for 6 to 17 wk. Control subjects ingested basal diets containing 0.4 mg fluoride per day, whereas experimental subjects received 10-mg fluoride supplements in divided doses with meals. Fluoride and calcium were measured in diets, urine, and feces. Serum analyses included calcium and ionic fluoride. Fluoride balances during both phases were uniformly negative in control subjects (mean -0.46 mg/day) but uniformly positive in supplement subjects (mean +2.58 mg/day). Calcium balances were markedly negative during bed rest in both groups. Serum fluoride concentrations increased proportionally to fluoride intake, averaging 0.016 ppm in the controls and 0.045 ppm in the supplement subjects. The supplement of 10 mg fluoride daily did not protect against bed rest-induced calcium loss, or cause any clinical or laboratory abnormality in any subject.