The factors that must be considered when preparing to implement a pharmacy information system are discussed. A major early decision is whether to locate the central processing unit (CPU) in the pharmacy department or the information services department. Other site considerations are the size of the CPU, environmental conditions, electrical needs, the presence of a backup CPU, and system security. System considerations include interfaces, daily management, preventive maintenance, other support functions, and printers. The system will affect all personnel in the pharmacy department; most jobs will have to be redesigned. Issues to address during implementation planning are forms and labels, the implementation schedule, benchmark development, system conversion, and problem resolution. Files must be developed with great attention to detail and accuracy, as they are the heart and soul of the system. Most departmental policies and procedures will have to be incorporated into the operations of the system. Other departments--in particular, information services, hospital purchasing, administration, nursing, and laboratory--may have requirements that should be considered. There may be legal requirements in the areas of drug order entry, file security and confidentiality, and retention of records. Extensive preparation and planning are necessary before a pharmacy information system can be implemented.