A guide was provided to the Registry of Toxic Effects of Chemical Substances (RTECS), especially for users of the electronic formats of RTECS. Information was provided on how chemicals were selected for inclusion, why chemicals may be excluded, and how toxicity information for RTECS was derived from the literature. Changes in RTECS since the last edition of the guide were discussed, including the addition of Beilstein Registry numbers to the Chemical Registry Number field, and a format change for OSHA Permissible Exposure Limits. The Department of Transportation Shipping and Labeling Regulation, the EPA Toxic Substances Control Acts Chemical Inventory status lines, National Toxicology Program data, and the International Occupational Exposure Limits have been updated. A detailed description was provided of the database, and record format. Tables provided RTECS abbreviations and their meanings, Toxic Effects Codes, Reproductive Effects Codes, route of administration abbreviations and definitions, species abbreviations and assumptions for toxic dose calculations. Sample RTECS records were provided in the formats of each of the online and CD/ROM vendors.