The inputs to mathematical models of library operations require periodic updating. The data collection effort necessary for such updating can be quite expensive; however, if the data collected can serve multiple purposes, the costs can perhaps be justified. A management information system (MIS) offers a vehicle for multiple uses of data, and thus offers an appropriate environment for implementation of models of library operations. The design of a model-based MIS is discussed in terms of three types of variables--mathematical/statistical, information processing, and human factors. General issues in these three areas are considered first, followed by a discussion of how these issues have been specifically addressed in the context of a prototype design of an online management information system for an interlibrary loan network. Considered in the discussion are MIS control, data base design, data entry, manager-MIS interaction, and MIS operating costs. Experience in modeling networks and in performing the work on which this report is based has led the authors to conclude that mathematical models can be successfully implemented. However, the question remains of whether or not a particular quantitative measure of work can be assigned to management information. The prototype model is currently operating at the University of Illinois on a DEC System-10. (Author/JPF)