We demonstrate how the mathematical treatment of the switching current data allows to characterize the variation of the domain structure during fatigue. Investigating PZT/YBCO heterostructures on SrTiO3and NdGaO3substrates we propose two different scenarios of fatigue-induced evolution of the domain structure. The essential role of as-grown domain structure is pointed out. It is shown that for higha-domain concentration the growth of the area occupied bya-domains prevails. For low “a-cratio” we obtain the competition between rejuvenation process and growth of the “frozen”c-domain area at the grain boundaries.