A gas chromatographic method for the assay of more than 5 ng/ml of fluoride in biological samples was established. By this method, the fluoride concentrations in monkey plasma, bloodcorpuscles and urine were measured. The fluoride concentration in plasma, which was the lowest before a meal (13-21 ng/ml), increased with time after the ingestions of diet and water. The value reached about twice (22-37 ng/ml) the initial value. Although the fluoride levels before and after a meal differed among animals, the level of each individual monkey was similar to that observed again a week later. The fluoride level in monkey blood-corpuscles was about half the plasma level. Total fluoride recovered in urine for 24 h amounted to 77 to 219 μg. The excreted amounts of fluoride, although they were different among the individuals, were similar in each individual to that observed again after a week. Fluorides in plasma and urine of rat and human were also determined, and compared with those of monkey. The control levels of fluoride in experimental animals determined in this study represent useful basic data for the determination of fluoride produced by metabolism of fluorine-containing drugs.