By means of mini-osmotic pumps implanted into rats (110-240 g body wt), steady-state fluoride levels in serum were raised throughout a 21-day period. Small volumes of a fluoride solution twice daily by gastric intubation produced transient elevations of serum fluoride, base-line values being reached within 6 h. The calculated mean fluoride increments in serum were similar to those observed in the infused rats through the daily dosages of fluoride were about 1.0 and 0.625 mg F/kg respectively. The amounts of fluoride incorporated into periosteal bone, incisor dentine and newly-formed enamel were generally greater in the infused than intubated rats, but these differences were reduced or absent when fluoride concentrations were related to the serum-fluoride concentrations. The results from a few rats given 1.25 mg F/kg per day by continuous infusion confirmed that the amounts of fluoride incorporated into mineralized tissues were closely related to the serum-fluoride levels. Rats were also given these fluoride supplements from the 10th day of pregnancy until parturition. The mode of fluoride administration did not affect the amount of fluoride incorporated into the tissues of the mothers or into the skeletal ash of the pups when the results were expressed in relation to maternal serum-fluoride levels.