A detailed analysis was made by the National Library of Medicine of the performance of the Medical Literature and Analysis System (MEDLARS) in relation to 300 actual "demand search" requests made to the systems in 1966 and 1967. The objectives of the study were : (1) to study the demand search requirements of MEDLARS users, (2) to determine how effectively and efficiently the present MEDLARS service is meeting these requirements, and (3) to recognize factors adversely affecting the performance of MEDLARS. A stratified sample of requests was used and for each relevant document citations already known to the requester were obtained. This information, as well as judgments from the requester about the relevance of documents retrieved by the MEDLARS search, was used to evaluate the results of the search. The test results showed that the system was operating, on the average, at about 58% recall and 50% precision, although few of the individual searches fall in the area bounded by the average ratios plus or minus 5%. Some recommended areas for improvement of this service are: user-system interaction, the MEDLARS index language, MEDLARS search strategies, use of foreign language material in MEDLARS, and continuous quality control of the operation. (CM)