Chlorine dioxide (ClO2) is under consideration as an alternative to chlorination as a disinfectant for public water supplies. A new assay was developed to qualitatively and quantitatively measure the metabolites of 36ClO2 in water and biological fluids. Studies in the rat indicated that the metabolites of ClO2 were Cl-, ClO-2, and ClO-b. At maximum absorption the percentage of ClO; formed in plasma from an initial oral dose of 10 mg/l 36ClO, was higher than from a 100 mg/l dose. The 36Cl concentration in washed packed cells indicated that 36Cl readily exchanges with chloride of saline.