This Guide was intended serve as a reference to the Registry of Toxic Effects of Chemical Substances (RTECS). It was prepared to assist the many users who access RTECS data through on line databases, compact disk or magnetic computer tape. Recent changes to the service were reviewed and a detailed description was offered of the various format selections. Each of the fields was defined and discussed, including the substance prime name, update, Chemical Abstracts Service Registry Number, RTECS number, molecular weight, molecular formula Wiswesser Line Notation, synonyms, compound descriptor codes, skin and eye irritation data, mutation data, reproductive effects data, tumorigenic data, acute toxicity data, other multiple dose toxicity data, cited references, reviews, standards and regulations, NIOSH standards development and surveillance data, and status. Tables included toxic effects code, reproductive effects code, routes of administration, species, master file data types, line numbers, line matrix for RTECS mutation data, line matrix for RTECS reproductive effects data, and the line matrix for RTECS lethality data.