The effect of mechanical relaxation on the relaxation behaviour of electrically conductive polymer composites was investigated. Experiments showed that filler content significantly affects the relaxation characteristics of the material due mainly to the effect of the filler type on the relaxation processes. Rubbers with active carbon blacks show ranges of slow and fast relaxation much more clearly than those with low-active carbon blacks, even at increased concentrations. With active carbon blacks, the increase in the filler-filler and polymer-filler interactions is balanced by the high content of low-active carbon blacks, because in both cases the elasticity modulus of the material and the internal friction during relaxation processes increase.