BIOMOD permits a designer to simulate a continuous system as he is developing it, to determine whether it performs as desired. He uses BIOMOD via an interactive graphics console comprising a television screen, data tablet, and keyboard. He represents his system by a hierarchical block diagram, each component of which is defined by another block diagram; analog-computer-like elements; algebraic, differential, or chemical equations; or FORTRAN statements--whichever terminology is most meaningful to him. When the designer completes the description of his system, BIOMOD generates an executable program, then runs system simulations under designer control. Displayed designer-specified curves are continuously and automatically updated during simulation. Simulations may be easily rerun after changing parameter values or the system design. This paper illustrates the application of BIOMOD to the design of an electromechanical control system. 11 pp. Ref.