For large perturbations the usual assumption in power reactor stability that the flux variations δn are small in comparison with the steady-state flux level n0 is not valid. Due to the product term of excess reactivity and neutron flux, harmonies are generated in the flux. The stability under such conditions can be discussed by extracting the fundamental flux component and obtaining a quasi frequency response as the ratio of fundamental incremental flux component to excess reactivity as a function of frequency and amplitude of perturbation. This approach is applied to two Argonne power reactors, the Experimental Boiling Water Reactor (EBWR) and the Experimental Breeder Reactor (EBR-I), where experimental frequency responses have been obtained and a peak has been observed. It is found that for both reactors the stability decreases as the amplitude of perturbation increases.