Age-related changes in membrane property are generally accepted phenomena. In this report, using the anti-lipoperoxidative measure of dietary restriction on membranes, the age-related effects of lipid peroxidation on membrane fluidity are explored in conjunction with the membrane-associated phospholipase A2. The fluidity of mitochondrial and microsomal membranes from ad libitum fed rats shows a progressive decline with age as indicated by a substantial decrease in two parameters, l/polarization and l/anisotropy. In contrast, the membrane fluidity of dietary restricted rats shows slight changes between 6 and 24 months of age. Evidence is presented to support the possibility that age-related peroxidation of membrane lipids may play a significant role in bringing about fluidity changes in aged membranes. The findings on the increased phospholipase A2 with age is also consistent with this conclusion.