Abstract:The correlation among lipid preoxidation, destruction of membrane system and ultrastruc- tural changes of mesophyll cells during salt stress was studied with the NaCl-resistant barley (Jian No. 4) and NaCl-sensitive wheat (Ning Mai No. 4) seedlings. During initial period of salt stress, mesophyll cells was able to maintain higher SOD activity, less lipid preoxidation and approximately integral membrane system; with the increasing intensity of salt stress, SOD activity lowered, MDA content increased obviously, electrolytes and UV-absorbed substances in cells leaked, cell organelles were destroyed severely, and even whole cell structure was ruined. The barley was able to maintain more intact cell structure and less damage of membrane system than the wheat did. These results indicated that changes of ultrastructure of mesophyll cells were consistent with disorder and damage of their membrane systems, and the damage of membrane system might be as a result of the increase of membrane lipid preoxidation.