Abstract： Objective To investigate the ultrastructure of enamel in the mandibular incisor tooth in a rat model of dental fluorosis.Methods Thirty 5-week-old male and female rats of SD strain were divided into three groups of ten.The animals in the control group were maintained for 8 weeks on pure deionized water,in the low-fluoride group deionized water with 22.5 mg/L of fluorine(50 mg/L NaF) was used,and in the high-fluoride group deionized water with 45 mg/L of fluorine(100 mg/L NaF) was used.All specimens examined were obtained from the mandibular incisors.Mid-sagittal ground sections were prepared from the direction of incisal point of each incisor for scanning electron microscopy(SEM) and atomic force microscopy (AFM).Results According to the SEM results,the prisms in the control group had a complete and clear column structure with closely packed enamel rods or enamel prisms.The prisms in the high-fluoride experimental group were collapsed and showed curved and fractured crystallites.The level of prism structural changes in the low-fluoride experimental group was in between that of the control group and the high-fluoride group.AFM for the middle layer enamel showed an average roughness(Ra) of (550.6±32.0) nm in the control group,(415.0±24.2) nm in the low-fluoride group,and (194.3± 11.3) nm for the high-fluoride group.There was significant difference between the high-fluoride group and the control group(P＜0.05).Conclusions Formation of rat enamel in the high fluoride environment presented obvious structural collapse and no clearance between prisms.Such changes appear to indicate a loss of normal prism structure in rat enamels from exposure to high levels of fluorine during the maturation period.