BACKGROUND Sodium chlorate occurs when drinking water is disinfected by chlorine dioxide. We studied the effects of sodium chlorate in rats and mice to identify potential toxic or carcinogenic hazards to humans. METHODS We gave groups of male and female rats drinking water containing 125, 1,000, or 2,000 milligrams (mg) of sodium chlorate per liter (L) of water for two years. Male and female mice received 500, 1,000, or 2,000 mg/L. Other groups of animals received plain tap water and served as the control groups. At the end of the study, tissues from more than 40 sites were examined for every animal. RESULTS Male and female rats receiving sodium chlorate had higher rates of follicular cell hypertrophy of the thyroid gland, and the groups receiving 2,000 mg/L had higher rates of thyroid gland cancer, compared with the control groups. Female mice exposed to sodium chlorate had a few pancreatic islet cell tumors. CONCLUSIONS We conclude that sodium chlorate caused some thyroid gland neoplasms in male and female rats. The pancreatic islet cell tumors in female mice may have been related to sodium chlorate exposure.