Objective To investigate the effects of fluo ride on thyroid structure in chicks.Methods 250 one-day chicks were randomly divided into 5 groups , 50 for each .Group I was the control ,and other four groups ,groupⅡ, Ⅲ,Ⅳ,Ⅴ were fed the diets containing 500,1 000,1 500,2 000 mg/kg of fluorineass odium fluoride, respectively, for 150 days. The contents of serum fluorine were determined, thyroid tissue slices were made , the weight of thyroids was weighed ,and the clinical manifestations of experimental chicks were observed. Results The animal models of ch ick fluorosis were successfully established.Compared with the control, the earlier changes of thyroid structure were that the relative weight of thyroid and thyroid follicle colloid in all NaF ingestion groups were less, and the size of thyroid follicles was smaller.During the later stage , the results just reversed to the earlier changes , I.e , both areas and diameters of thyroid follicle were larger and the colloid was over filled , and the follicular epithelia were obviously flatted in the enlarged follicles for being crushed by over-filling coll oid. Moreover, hyperplastic nodules, consisting of thyroid parafollicular cells appeared in group V.Conclusions Fluoride can seriously damage thyroid structure . During the earlier stage , fluoride can in duce thyroid atrophia , however, during the later stage ,it can induce thyroide nlargement which is nodular and colloid goiter.