The purpose of this document is to present a final description of the original MEDLARS system as it evolved through four years of operation. The system is described as it was functioning on January 1, 1968. Among the various system elements discussed are: (1) the input subsystem, including journal selection and coverage, Medical Subject Headings (MeSH), and indexing; (2) the retrieval subsystem, including request analysis, search formulation, file search, and printout of retrieved citations; and (3) the publication subsystem, including the MEDLARS photocomposer and computer programs for producing MEDLARS publications. A summary of operating experience for the period includes discussion of system problems, changes, and evaluation. Also discussed is the extension of MEDLARS technology to the cataloging of books and serial titles. This system produces two major products: (1) catalog cards for the central NLM card catalog and (2) the NLM current catalog, a computer-produced book-form catalog available to other medical libraries on a current and frequent basis for use as an acquisitions and cataloging tool. Appended is a list of approximately 200 selected references. (JW)