The first and rate limiting step in the conversion of linoleic and alpha-linolenic acid is catalyzed by the delta - 6 - desaturase (D6D) enzyme. Rat liver microsomal D6D activity decreases on linolenic acid at a rate proportional to the animal age; on alpha-linolenic acid the decrease in D6D activity begins only later than on linoleic acid. The fatty acid composition of liver microsomes determined by gas chromatographic analysis confirms the impairment of the enzymatic activity directly measured. Our data indicate a correlation between aging and D6D activity impairment. The loss of D6D activity may be a key factor in aging through altering the eicosanoid balance.