Dietary restriction was used in this study as a modulator of free radical reactions to examine the effects of age on the physico-biochemical property of synaptosomal membranes. Synaptosomal membranes were isolated from the frontal cortices of 6- and 24-month-old barrier-reared male Fischer 344 rats maintained on either an ad lib (AL) or a 40% diet restricted (DR) feeding schedule. The age-related production of reactive oxygen species (ROS) was only seen in the AL group, and dietary restriction suppressed the amount of the reactive species at both ages. Although membrane fluidity significantly decreased with age in AL fed rats, no change occurred in DR rats. Because age-related increases in cholesterol/phospholipid occurred in both AL and DR groups, fluidity loss may be influenced by factors other than cholesterol. We suggest that lipid peroxidation may be a major factor in the change in fluidity during the aging process.