Lipid peroxidation and membrane-related enzyme changes in the cerebral cortex of stroke-prone rats (SHRSP) and normotensive rats were examined at 5 and 20 weeks of age. 2. In vivo formation of thiobarbituric acid-reactant substances was higher in SHRSP at 20 weeks of age and in vitro generation of free malondialdehyde was greater in SHRSP brains, both at 5 and 20 weeks of age, as compared with those in WKY. 3. Membrane-associated enzymes such as Na/K-ATPase and 5'-nucleotidase activities were lower in 20-week-old SHRSP than in age-matched WKY. 4. These results indicate how very prone the SHRSP brain is toward lipid peroxidation and subsequent membrane-related enzyme changes.