A number of soluble and membrane associated enzymes of glycolysis, pentose phosphate pathway and other related enzymes were measured in three different brain regions during aging. Enzymes utilizing and synthesizing peroxides were also included. Increasing levels of peroxidative products are known to accumulate in the brain with age. The membrane associated enzymes were found to be the primary focus of damage. Phosphofructokinase and glucose-6-phosphate dehydrogenase exhibited an unusual pattern when measured in whole homogenates. A progressive decrease in the synaptosomal bound hexokinase was found with increasing age. The synaptosomal phosphofructokinase (PFK) also showed a significant decrease with aging. Significant decrease in the incorporation of myoinositol into phospholipids and a loss of activity of membrane bound adenylate cyclase with age indicated that changes must be occurring in the structure of the brain and the loss of cerebral competence in the senescent brain may arise from peroxidative damage to membranes.