This is the fourth in a series of eight reports of a research study for the National Agricultural Library (NAL) on the effective utilization of bibliographic data bases in machine-readable form. It describes the general functional and software requirements of an NAL system using external sources of bibliographic data. Various system design alternatives are compared and user requirements are discussed. It is recommended that studies be undertaken to (1) establish quantitative specifications of various user needs, especially in regard to response time and query volume, and (2) analyze the data bases external to NAL to determine the quantity of pertinent information contained in them and to evaluate the quality or nature of the indexing techniques used. This information will have a critical effect on the choice between a remote off-line type of system (using little or no data from external sources) and an integrated system using substantial amounts of external data converted to NAL format and using an integrated, controlled NAL vocabulary. The third alternative, a combined system using the uncontrolled external indexing vocabularies, is not considered to be feasible. (JW)