Two day-old mouse first molar tooth germs were grown in medium containing sodium fluoride for up to 4 days. Dose levels ranged from 200 ppm fluoride down to 0.1 ppm. A dose of 100 ppm was lethal to tooth germs. Doses from 50 to 5 ppm resulted in consistent production of chains of large clear vacuoles in the supranuclear region of secretory ameloblasts together with large dark staining globules in the apical one third to one half of these cells. Enamel matrix secretion was arrested within this dose range as evidenced by the presence of only a very thin layer of enamel matrix in sections as compared with a thicker layer in similar areas of controls. Presecretory ameloblasts were not visibly affected by fluoride. Some variations in these effects were observed at 2 and 1 ppm fluoride. Tooth germs treated with 10 ppm for 48 hours, then grown in control medium for 4 days, underwent reversal of characteristic fluoride effects with resumption of enamel secretion. One ppm appeared to be the lower limit for production of these effects as observed with the light microscope. The cytology of fluorotic ameloblasts agreed with that of in vivo fluoride studies. Indications are that this in vitro system is promising for study of fluoride effects on developing teeth. Its advantages are discussed.