The authors describe the development of an automated system to perform writing of prescriptions and associated advice. A prototype computer program has been in operation in a dermatologist's office for three years. All prescriptions are routinely generated using the system. The program, originally developed for a multiuser computer system, now operates in a self-contained personal computer. The program is constructed to use external human readable files as the drug formulary or database. Experience with this system and efforts to expand its applicability to another ambulatory care setting (gastroenterology) are described. Rapid access to drug names and related information is achieved using diagnosis-specific subsets contained in a personally defined formulary. This organization supports rapid pruning of the list of drugs. From this point, a prescription of the selected drug is displayed on a video terminal. The base level prescription is easily modified using only five keys organized in a cluster. Preliminary training and performance studies are summarized.