This study was undertaken to investigate the influence of age and dietary linolenic acid content and the linolenic acid/linoleic acid (LAN/LA) ratio on the brain lipid composition and membrane-bound enzyme, acetylcholinesterase(AchE) activities. AchE was selected as a test case for the relationship between cell lipid composition and cell membrane function. The male rats were fed diets with 0.2, 0.4, 0.6 of LNA/LA ratio within 8% LNA(H-LNA) or 4% LNA(L-LAN) of total fatty acid content for different feeding period(1, 4, 12 month). The fats used s source were sesame oil, perilla oil, soybean oil and beef tallow. The AchE activity of brain crude synaptosomal fraction was reduced with advancing age, showing 20-30% reduction in 12M compared with 1 M, and the P/C ratio was reduced in old rats. In 1 and 4 monthed rats, AchE activites was higher in H-LAN-0.2 and L-LNA-0.2 and 0.4 group. In accordance with rising of AchE activities was higher in H-LNA-0.2 and L-LNA-0.2 and 0.4 group. In accordance with rising of AchE activities, the PC/PE ratio increasedin those groups. Paricularly in L-LNA, the PC/PE ratio increased as the AchE activites for decline of membrane fluidity with increasing cholesterol and decreasing P/C ratio when rats were old. Also, AchE activity increaed with increasing PC/PE ratio which depended on the dietary LNA/LA ratio within each LNA content. Therefore, it is concluded that the lipid composition of cell membrane influenced the AchE activiteis, which was mediated by aging and the modification of dietary LNA/LA ratio.