By using a perturbation approach, we theoretically investigate nonlinear ac susceptibilities of dynamic electrorheological fluids under a do electric field. We consider the effect of a finite volume fraction. It is shown that the dynamic effect arising from a shear flow plays an important role in the ac responses. Meanwhile, the effect of a finite volume fraction offers a correction. The collective relaxation time is also derived by using our recently-established dielectric dispersion spectral representation, and it is shown to generally increase with the volume fraction of particles. Some theoretical results are compared favorably with the experimental observations by others. (c) 2006 Elsevier B.V. All rights reserved.