The fatty acid composition of total lipids and individual phospholipids and the ratio of cholesterol-to-phospholipid content were studied in cerebral cortex, subcortical white matter, cerebellum and medulla oblongata/pons in 4-, 21.5- and 28-month-old rats. The cholesterol-to-phospholipid molar ratio in subcortical white matter, medulla oblongata/pons and cerebellum in 28-month-old rats was found to be 17, 17 and 16% higher, respectively, than in adult rats. These alterations in the molar ratio were produced as a result of a net increase in cholesterol content rather than by changes in the total phospholipid content. The content of alkenylacylglycerophosphoethanolamine (alkenylacyl GPE) increased in 28-month-old rats with respect to 4-month-old rats, following the order cerebral cortex > cerebellum > medulla oblongata/pons > subcortical white matter. The fatty acid composition of total lipids showed an increase in monounsaturated fatty acids (MUFA) and a decrease in polyunsaturated fatty acid (PUFA) content with increasing age in all regions studied, such changes being more marked in cerebellum, medulla oblongata/pons and subcortical white matter than in cerebral cortex. The proportion of 22:6(n-3) in cholineglycerophospholipids in the different brain regions of 28-month-old rats showed a slight decrease with respect to that in adult rats following the order cerebellum > medulla oblongata/pons > subcortical white matter, whereas that of 20:4(n-6) decreased only in cerebellum. Ethanolamineglycerophospholipid fatty acid composition was modified in 28-month-old rats through a marked increase in monounsaturated fatty acids (18:1(n-9) and 20:1(n-9) specifically) in all brain areas. The MUFA content of alkenylacyl GPE increased with increasing age in all the regions studied, in the order cerebral cortex > medulla oblongata/pons > subcortical white matter > cerebellum. An increase in the MUFA content of diacylglycerophosphoethanolamine was observed in medulla oblongata/pons and cerebellum of aged rats with respect to control rats. PUFA content of alkenylacyl GPE decreased mainly in medulla oblongata/pons, cerebral cortex and subcortical white matter of aged rats. The PUFA content of serineglycerophospholipids was the most affected by aging. Changes occurred mainly in the content of 22:6(n-3), 22:4(n-6) and 20:4(n-6) in cerebellum and of 22:6(n-3) and 22:4(n-6) in subcortical white matter. Changes in the inositolglycerophospholipid fatty acid content of 28-month-old rats were observed mainly in medulla oblongata/pons, which showed a decrease in PUFA (22:4(n-6) and 22:6(n-3)) content and an increase in MUFA (18:1 and 20:1).