Since the late seventies Flexible Manufacturing Systems (FMS) have become increasingly popular in manufacturing. A search of the literature by Gunasekaran et al. (1993) shows that there is an abundance reported research on FMS design, justification and operational issues. However, only very few papers deal with the practical implications of existing manufacturing resources planning systems like MRP on the production planning of FMS (see e.g. Mazzola (1989) and Mazzola et al. (1989)). In this paper we discuss an open architecture adapted and the development of a production planning decision support system for the integration of FMS production planning and MRP systems. The paper is based on a real life study of a manufacturer of diesel engines for ships and stationary applications in the Netherlands.