To eliminate oscillatory instability in electric power systems by inserting stabilizers, it is important to have available analytical criteria for the choice of the most suitable generators to which to put the stabilizing feedbacks, and for defining the most convenient parameters for them, without resorting to simulation. Applying some properties of control systems it is possible to set up digital programs based on calculations of eigenvalues and eigenvectors that solve these problems satisfactorily. The report then shows application of the procedures thus defined to a practical case of instability, and field test results that confinn the elimination of instability.