The fitting of mathematical models to physiological systems can be tedious and difficult, whether one uses analog or digital computer methods. Both methods have their pros and cons depending on the available hardware and software and on the type of modeling. In recent years many digital simulation languages have been written combining analog-like and digital features to facilitate modeling, but, for a variety of reasons, none of these was suitable for our applications. We, therefore, designed a new digital simulation control system, SIMCON, which is described in this paper.The primary objectives were to provide: Maximum man-machine interaction at run-time, including visual displays, digital control, and both continuous analog and digital parameter adjustmentThe ability to generate solutions and to fit them to experimental data or other theoretical curves with a minimum of computer memoryThe option to use a mathematically oriented language, FORTRAN, and block operators with variable input/output.The result is a relatively general and simple simulation system which is easy to use and has wide versatility.