BACKGROUND Dibromoacetonitrile occurs in chlorinated drinking water that contains bromine and interacts with organic matter.We studied the effects of dibromoacetonitrile in drinking water on male and female rats and mice to identify potential toxic or cancer-related hazards. METHODS We gave drinking water containing 50, 100, or 200 milligrams of dibromoacetonitrile per liter of water to groups of 50 male and female rats and mice for two years. Control animals received the same tap water with no chemical added. At the end of the study, tissues from more than 40 sites were examined for every animal. RESULTS Survival was similar for animals receiving dibromoacetonitrile and the controls. Male and female rats receiving dibromoacetonitrile had increased rates of squamous cell adenomas or carcinomas of the mouth (oral mucosa or tongue). Two male rats receiving 200 mg/L had rare adenomas of the glandular stomach. There were a few instances of skin tumors in female rats receiving 100 or 200 mg/L. Male and female mice exposed to dibromoacetonitrile had increased rates of squamous cell papillomas of the forestomach. There were also slight increases in the occurrence of liver tumors in male mice. CONCLUSIONS We conclude that dibromoacetonitrile in the drinking water caused cancer of the oral cavity in male and female rats and of the glandular stomach in male rats. We conclude that dibromoacetonitrile caused cancer of the forestomach in male and female mice. Tumors of the skin in female rats and of the liver in male mice may also have been related to exposure to dibromoacetonitrile.