The situational and personal factors affecting the usage of a computer based information service by health care professionals are described. Of 166 health care professionals who were identified as potential users of MEDLINE at the University of Connecticut, 126 carried out one or more online searches. Pre-use and post-use questionnaires were administered to solicit information in the following areas: job profile, information needs, current use of information sources, satisfaction with information sources, experience with automated information systems, beliefs about computers and automated information systems, physical difficulty in using the system, and type of MEDLINE training received. It was concluded that the relevance of the information available to the immediate job-related problems of the user is the major determinant of the first use of the service. Total information needs, dissatisfaction with other sources of information, and beliefs about the satisfying nature of computer interaction and the reliability of computer machinery are predictive of more frequent use. This report is part of a National Institutes of Health (NIH) project carried out at five locations in the United States. (Author/THC)