The document provides the health effects basis to be considered in establishing the MCLG. To achieve the objective, data on pharmacokinetics, human exposure, acute and chronic toxicity to animals and humans, epidemiology, and mechanisms of toxicity were evaluated for diquat. Specific emphasis is placed on literature data providing dose-response information. Thus, while the literature search and evaluation performed in support of the document was comprehensive, only the reports considered most pertinent in the derivation of the MCLG are cited in the document. The comprehensive literature data base in support of the document includes information published up to April 1987; however, more recent data have been added during the review process and in response to public comments.