The document provides the health effects basis to be considered in establishing the MCLG. To achieve this objective, data on toxicokinetics, and acute, subchronic and chronic toxicity to animals and humans are evaluated for nickel. Specific emphasis is placed on data published in peer-reviewed literature providing dose-response information. Thus, while the literature search and evaluation performed in the development of the document have been 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 1990; however, more recent data may have been added during the review process. The risk assessment of nickel was reviewed by the Science Advisory Board in August, 1990. The document is consistent with their recommendations.