Chlorine dioxide (ClO 2 ) is an effective drinking water disinfectant, but sodium chlorate (NaClO 3 ) has been identi ed as a potentially harmful disinfection by-product . Studies were performed to describe the developmen t of thyroid lesions in animals exposed to NaClO 3 in the drinking water. Male and female F344 rats and B6C3F 1 mice were exposed to 0, 0.125, 0.25, 0.5, 1.0, or 2.0 g/L NaClO 3 for 21 days. Additional male F344 rats were exposed to 0, 0.001, 0.01, 0.1, 1.0, or 2.0 g/L NaClO 3 for 90 days. Female F344 rats were exposed to 0, 0.5, 1.0, 2.0, 4.0, or 6.0 g/L of NaClO 3 for 105 days. Thyroid tissues were processed by routine methods for light microscopi c examination, and follicular cell hyperplasia was diagnosed using a novel method. Thyroid hormone levels were altered signi cantly after 4 and 21 days. NaClO 3 treatment induced a concentration-dependen t increase in the incidence and severity of thyroid follicular cell hyperplasia. Male rats are more sensitive to the effects of NaClO 3 treatment than females. Follicular cell hyperplasi a was not present in male or female B6C3F 1 mice. These data can be used to estimate the human health risk that would be associated with using ClO 2 , rather than chlorine, to disinfect drinking water.