In this study, our objective was to determine blood T3, T 4 and TSH levels in rabbits, which were administered fluoride chronically. Twenty one male rabbits were divided into three groups. The rabbits in group I (control), group II and III were administered, respectively, normal drinking water (containing 0.07 ppm fluoride), drinking water containing 10 ppm fluoride and water containing 40 ppm fluoride for 70 days. On the days 21 and 70 blood was taken and plasma T3, T4, TSH and fluoride levels were studied. According to the results obtained a statistically significant increase was observed in the T3 level of the rabbits receiving fluoride at 40 ppm level for 70 days compared with controls, and a statistically significant decrease was detected in T4 level. Plasma fluoride level increased in proportion to the amount and duration of the fluoride administered.