Early models of mammalian metabolism were relatively simple, and frequently linear (1, 2). In many cases simplification required the lumping of two or more morphologically distinct organ level unit processes as a single, quasi-linear process, thus making physiological interpretation of simulation results difficult. With the introduction of versatile digital simulation tools during the last decade, models appeared which were increasingly isomorphic with in vivo system (3,4), and which included some of the simpler nonlinearities (e.g., thresholds, saturations) which govern the dynamic range of the system components.