In a previous study from this laboratory, chlorine dioxide (ClO2) treated drinking water depressed thyroxine (T4) levels in the African green monkey. The present study again demonstrated a decrease in T4levels in the same species after 4 wk of oral exposure. However, after 8 wk of treatment T4levels rebounded to above pretreatment levels, coinciding with an increase in thyroid radioiodide uptake. This T4rebound phenomenon and increased iodide uptake may be due to a compensatory endocrinological mechanism. In rats, T4levels dropped during the 8‐wk ClO2treatment period in a dose‐dependent manner, and no rebound effect was observed. Iodide uptake values in the rat were not affected. It appears that ClO2may have an effect on thyroid function in both species.