Fluoride pharmacokinetic data are presented for infants given a fluoride supplement. Seventeen infants participated in a total of 20 studies. On one day, 0.013 mmol (0.25 mg) fluoride was given as a supplement (fluoride supplement study), and on another day a placebo was given (control study). Samples of plasma and urine were collected for 5 h and analyzed for fluoride. During control studies fluoride intake averaged 0.15 mumol/kg (2.9 micrograms/kg), and plasma fluoride concentrations ranged from 0.05 to 0.11 mumol/L (10 to 20 micrograms/L). In nine instances, the quantity of fluoride excreted in the urine was more than twice that consumed. When the fluoride supplement was given, total fluoride intake averaged 1.93 mumol/kg (36.6 micrograms/kg). Plasma peak concentration was reached by 30 min in 14 studies and by 60 min in six studies. Mean plasma peak fluoride concentration was 3.3 mumol/L (63 ng/mL). Area under the plasma concentration curve averaged 236 nmol.m-1 x min (4479 ng.mL-1 x min) and was not related to the dose of fluoride. The rate of urinary excretion was significantly correlated with rate of urinary flow. When the dose of fluoride was expressed per unit of body weight, fluoride retention was strongly related to the dose. Retention of the fluoride absorbed from the fluoride dose ranged from 75.4 to 87.6%. Plasma clearance averaged 6.8 mL.kg-1 x min-1 and decreased significantly with age. Net fractional clearance (renal clearance of the fluoride dose/GFR) averaged 56.7%, which was significantly greater than the 29% observed during the control studies. The greater percentage retention of fluoride by infants than by adults is probably explained by a greater capacity of the infant to deposit fluoride in hard tissues.