YBa2Cu3O7-x compound (T-C = 90 K) was milled in a rotary mill for up to 30 h. Study of the thermal behaviour by differential scanning calorimetry revealed one exothermic peak in the temperature interval up to 400 degrees C with the highest rate of heat release at T-max. The thermal effect increased with the time of milling. The shift of T-max with different heating rates (beta) was measured for the rates 5, 10, 20 and 40 degrees C min(-1). The linear dependence In (beta/T-max(2)) versus 1/T-max is typical for an Arrhenius process. The determined activation energy of heat release E = 1.32-1.67 eV (127.36-161.13 kJ mol(-1)) corresponds to the activation energy of atomic oxygen diffusion in the YBa2Cu3O7-x lattice. Therefore this effect is related to the defect relaxation through diffusive redistribution of oxygen atoms.