The Registry of Toxic Effects of Chemical Substances (RTECS) is a database of toxicological information compiled, maintained, and updated by NIOSH. RTECS is a congressionally mandated activity established by Section 20(a)(6) of the Occupational Safety and Health Act of 1970 (PL 91-596). The original edition, known as the "Toxic Substances List," was published on June 28, 1971, and included toxicological data for approximately 5,000 chemicals. Since that time, the list has continuously grown and been updated, and its name changed to the current title, "Registry of Toxic Effects of Chemical Substances." RTECS now contains over 148,000 chemicals an NIOSH strives to fulfill the mandate to list "all known toxic substances... and the concentrations at which... toxicity is known to occur". Many consider RTECS the premier toxicological database. RTECS is a compendium of data extracted from the open scientific literature. The data are recorded in the format developed by the RTECS staff and arranged in alphabetical order by prime chemical name. Six types of toxicity data are included in the file: (1) primary irritation; (2) mutagenic effects; (3) reproductive effects; 94) tumorgenic effects; (5) acute toxicity; and (6) other multiple dose toxicity. Specific numeric toxicity values such as LD50, LC50, TDLo, and TCLo are noted as well as species studied and route of administration used. For each citation, the bibliographic source is listed thereby enabling the user to access the actual studies cited. No attempt has been made to evaluate the studies cited in RTECS. The user has the responsibility of making such assessments.