In view of the recommendation that fluoride supplements via drinking water or table salt prevent dental caries, we analyzed whether fluoride had antithyroid properties in a sensitive experimental model. Rats were given either 60 or 200 micrograms/ml fluoride in the drinking water. This raised the serum fluoride concentration from 0.165 to 0.246 in the first and to 0.576 micrograms/ml in the second instance. Although the higher fluoride dose was near the toxic range, no antithyroid effect was observed. Neither organification of iodine, nor any subsequent step of thyroid hormone biosynthesis (transformation of monoiodotyrosine to diiodotyrosine and then to thyroxine) were affected. Fluoride had no effect on thyroglobulin content of the thyroid gland or on the degree of iodination of thyroglobulin.