The goal of this project is to provide support of National Toxicology Program (NTP) hazard identification activities targeted toward the prevention of diseases or adverse effects caused by environmental exposure to chemical or physical agents. Toxicity testing is an important aspect of public health research in that it serves to identify chemicals that are hazardous to human health. Proper conduct of toxicology studies is required to ensure that the resulting data are reliable. This project involves the conduct of toxicity studies of pentavalent (sodium metavanadate) and tetravalent (vanadyl sulfate) vanadium compounds in rats and mice, following exposure in drinking water. These compounds are found in drinking water and thus human exposure is anticipated via this route. The data from this project will be used in the development of sound, scientific conclusions about the potential toxicity of vanadium in rats and mice and ultimately supports the risk assessment efforts of National Toxicology Program and other federal agencies. In addition, the public can be adequately informed about risk factors arising from exposure to chemicals containing pentavalent and tetravalent forms of vanadium. Keywords: toxicity, drinking water, vanadium, pentavalent, tetravalent, vanadyl sulfate, sodium metavanadate.