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Abstract-Ferroelectric perovskites, such as Pb(Ti,Zr)O, and BaTiO,, are subject to ageing effects, caused by a gradual stabilization of the ferroelectric domain structure. We studied the development of this stabilization in Mn-doped BaTiO, monocrystals having a special kind of domain structure. We concluded from measurements of the growth of the internal bias field E, and the change of the sideways 180" domain-wall mobility with time, that the fairly strong stabilization in this material originates from volume effects, which implies a gradual reorientation of polar defects with respect to the direction of the spontaneous polarization, both 180" domain-wall pinning and surface-layer effects being completely absent. A proposal is made about the nature of these defects.