The internal stress depends on strain rate, temperature and grain size and it is the major part of the applied stress in region II. The sigma //i/ sigma ratio depends more strongly on temperature than grain size in region II. The strain rate sensitivity, grain size exponent, and activation energy, based on the measured effective stress in region II are 0. 82, 2. 8 and 33. 5 kJ/mol, respectively. These data are in agreement with the Ashby-Verrall model of superplasticity. In region III, internal stress decreases to a small fraction of the applied stress with increasing temperature.