Trienoic fatty acids, namely 0~-linolenic acid and hexadecatrienoic acid, present in leaf lipids are produced by (0-3 fatty acid desaturases located in the endoplasmic reticulum and plastid membranes. The changes in the level of trienoic fatty acids during leaf maturation were investigated in wild-type plants of Arabidopsis thaliana (L.) Heynh. and in the fad7 mutant deficient in the activity of a plastid (0-3 desaturase. The levels of trienoic fatty acids increased in 26 ~ and 15 ~ wild-type plants with maturation of leaves. The increase in trienoic fatty acids was mainly due to galactolipids enriched in plastid membranes. In addition, the relative levels of trienoic fatty acids in major glycerolipids, including phospholipids enriched in the endoplasmic reticulum membranes, also increased with leaf maturation. By contrast, when the fad7 mutant was grown at 26 ~ the relative levels of trienoic fatty acids in individual lipids decreased with leaf maturation. The decreases in the levels of trienoic fatty acids, however, were alleviated when the fad7 mutant was grown at 15 ~ These results suggest that the plastid (0-3 desaturase plays a major role in increasing the levels of trienoic fatty acids with leaf maturation.