This paper describes the design and implementation of an on-line medical data system for general practitioners which was developed in a joint effort by the Institute of Biometry and Community Medicine in Exeter, the surgery of Dr. Preece, and IBM United Kingdom. The system is designed to permit general practitioners to create, access, and maintain centrally-stored clinical records via remote terminal devices. The pilot version of this retrieval system was established in the city of Exeter, Devon, where three terminals and control equipment in a surgery were connected by telephone lines to an IBM 360-40 data processing system at Croydon, Surrey, 200 miles away. During the trial period, the users have found that by employing computerized medical recordkeeping, day-to-day patient management proved superior to the conventional paper record organization. Experience gained from the current prototype system will be used in larger-scale experiments in which family doctors and hospitals will be linked into a computer network with a central bank of clinical documents and patients' records under the Devon and Exeter Health Services Computer Project. © 1971.