Effect of heat (37degreesC, 24 hours) and chilling (4degreesC, 24 hours) stress on SOD activity was investigated in shoots and roots of hexaploid and tetraploid wheat cultivars. In order to analyse the changes of SOD isozymes against heat and chilling stress, shoot and root extracts were subjected to native PAGE. Extracts of both tissues revealed the presence of Mn and Cu/Zn isozymes of SOD. Cu/ZnSOD activity comprised 90% of total SOD activity in both tissues. Heat stress caused a significant increase in SOD activity both in root and shoot tissues of the hexaploid cultivars whereas a decrease was observed in the tetraploid cultivar. Under cold stress no significant change in SOD activity was observed in shoot tissues of all cultivars. On the contrary, SOD activities in the roots of the hexaploid cultivars were significantly enhanced while a decrease was observed in the roots of tetraploid cultivar. Our data suggest that enhanced SOD activity might be one of the reasons of relatively greater stress tolerance of bread wheat cultiars over durum cultivars under heat and chilling stress conditions.