A system is described that greatly facilitates inter active discrete-time simulation in FORTRAN. The system allows the user to alter and examine the value of any variable during execution. It permits the user to plot any variable as simulation proceeds and to save all variables during simulation. Once a simulation run is complete, the user may go back and look at all previous values of all variables. This system permits the user to restart his simulation at any previous time, with all variables reset to their former values. Many other features are provided to enable novice users to control the simulation with a minimum of external aid. All features are called by simple commands issued during the simulation by the user, or predefined by him in a data file. The features of the system are largely machine-indepen dent and could be easily converted to any time- sharing or batch-processing system. The programming details of the control system are not included, but a general outline of the concepts of its operation are given.