An object-oriented (OO) approach to developing a computer simulation of a complete pastoral dairy farm system is outlined. Since component models already existed which encapsulated the functionality of the high level objects in the overall system it was a primary design goal to be able to incorporate them into the OO framework. These components had been developed in procedural programming languages and techniques were investigated to incorporate them with little or no re-coding, and which allow them to be used in an ‘object-like’ way. The techniques involved recompilation of legacy source-code as dynamic link libraries (DLL's), and accessing stand-alone executable programs through inter-process communication (IPC) protocols. A cow metabolism model developed in the proprietary ACSL environment, and a pasture growth model for which FORTRAN source code was available were incorporated into the Smalltalk OO framework using the Microsoft Component Object Model (COM) techniques. Multiple instances of each component (each representing a distinct real farm entity) were required by the OO framework and the COM methodology was able to support this. The integration of these components into the host OO framework involved using proxy objects to provide translation to/from the COM messaging, thus providing the OO framework with consistent inter-object communication.