The aging characteristics of stator bars, which have been subjected to accelerated aging tests in a three-phase model stator under simultaneously applied electrical, mechanical, and thermal stresses, have been evaluated using diagnostic test procedures. The ac breakdown test has been found to be the most effective means for assessing the extent of aging undergone by the stator bars. The results indicate that under multistress conditions relatively small increases in the magnitudes of the stress acceleration factors are sufficient to lead to significant increases in the aging rate of the insulating systems of stator bars.