In this talk we present non-perturbative studies on the coherent ac response of quantum dot-reservoir systems in the presence of strong many-body correlations. The Coulomb repulsion and the spin exchange coupling in the quantum dot are found to break the universality in the charge relaxation resistance which was previously derived under the general single-particle picture. We found that the charge relaxation resistance has peaks at the relevant Kondo temperature and that the zero-frequency resistance is considerably enhanced when the Zeeman splitting is comparable to the Kondo temperature. Since the Kondo effect depends highly on the spin correlations existing in the dot, diverse features are also observed near the singlet-triplet transition. We explain the observed enhancement of the relaxation resistance in terms of the generation of particle-hole excitations in the reservoir accomplished by spin-flip processes in the dot and discuss the relation between the resistance and the spin fluctuations.