An apparatus for the power stabilization of medium‐level microwave power sources, intended for laboratory use, by means of a degenerative feed‐back network connected in tandem with the power source is described. An analysis of the operation of the network is presented, based on obtaining optimum stabilizing action and optimum transmission efficiency for the network. Design of an electrically‐operated attenuator (which is a servo device), and its effect on the stabilization characteristics of the network are discussed. A transmission efficiency of 70 percent and a stabilization factor of 21 were obtained in an experimental model.