The viscoelastic behaviors of extra-heavy crude oils were investigated using a dynamic rheometer. The spectromechanical analyses of the fluids showed the presence of a relaxation process on the storage modulus curve. Both position and amplitude of this phenomenon were strongly dependent upon the value of the controlled stress. In particular, an increase of the applied stress induced a reduction of its average relaxation time. Similar effects could also be obtained with a temperature rise. The mechanical relaxation phenomenon was interpreted as being characteristic of the spatial rearrangement of interacting asphaltene aggregates. The global structure could be dislocated provided that the stress exceeded a critical value as predicted by theoretical approaches based on the potential gap concept. The original conformation of the asphaltene aggregates could be recovered under precise conditions of stress and time.