The Wigner‐Dyson distribution of level spacings plays a central role in the classification of quantum chaotic systems. However, this description provides no information about the dispersion of the energy levels,Ei(X) in response to an external perturbation. We demonstrate that the response depends on the detailed properties of the system only through mean‐level spacing, and a conductance which we define. We derive a non‐perturbative expression for the autocorrelator of density of states fluctuations for systems taken from orthogonal and unitary ensembles, showing that the dependence on these two parameters can be removed by a rescaling. We argue that this description applies quite generally to arbitrary perturbations, and that after rescaling the statistical properties of the random functions,Ei(X) become universal.