Several studies have demonstrated that inhibitors of cyclooxygenase (COX) attenuate various neuronal injuries and age-dependent demented conditions. From these findings, we proposed to test the effect of age on COX activity and its possible suppression by the antiaging action of dietary restriction in the rat brain. The status of reactive oxygen species (ROS) was also assessed to correlate with COX activity to delineate the underlying mechanism of the altered COX activity during aging. These results showed that COX activity significantly increased in 24-month-old rats compared with 6-month-old rats in an ad libitum group. Interestingly, mRNA and protein levels of COX-2 showed little corresponding age-related change. The formation of ROS was found to increase gradually with age in ad libitum fed rats. However, dietary restriction suppressed the increase at the age of 24 months. To substantiate the relationship between ROS and COX activity when the rats were 24 months of age, we conducted in vitro experiments with a C6 glioma cell line. Together, it is concluded that increased COX activity with age is due to the activation of COX catalytic reaction by ROS without increased gene expression of COX-2 and that it is related to the increased pro-oxidant status in aged rats.