*MEDLEARN*, a second-generation computer-assisted instruction (CAI) program available (nationally) since October 1976, provides on-line training for MEDLINE, one of the National Library of Medicine's (NLM) Medical Literature Analysis and Retrieval System (MEDLARS) data base. *MEDLEARN* was developed as a joint effort between NLM and The George Washington University Medical Center. Using MEDLINE formats throughout, *MEDLEARN* combines tutorial dialogue, drill and practice, testing, and simulation. The program was designed in three tracks oriented to basic methods, advanced techniques, and new developments. Each topic is presented on two levels, permitting an alternate explanation for users encountering difficulty. *MEDLEARN*, coded in the computer language PILOT, was developed with a modular structure which promotes ease of writing and revision. A versatile control structure maximizes student control. Frequent interactions check immediate recall, general comprehension, and integration of knowledge. Two MEDLINE simulations are included, providing the student an opportunity to formulate and execute a search, have it evaluated, and then perform the search in MEDLINE. Commenting, news broadcasting, and monitoring (with permission only) capabilities are also available. Subjective field appraisals have been positive and NLM plans to expand *MEDLEARN* and produce similar programs for other data bases.