Mitochondria are especially important in cellular senescence since reactive oxygen species (ROS) have been found to be generated constantly as an endogen threat. On the other hand, mitochondrial defence depends mainly on glutathione (GSH), superoxide dismutase (SOD) and glutathione peroxidase (GP(x)). In this study, liver and brain mitochondria of 6- and 22-month-old rats were investigated. Liver mitochondrial malondialdehyde (MDA) levels were found increased, whereas mitochondrial GSH was decreased. SOD activity has been observed elevated, whereas GP(x) was unchanged. However, brain mitochondrial MDA, GSH and SOD and GP(x) activites were found unchanged in old rats. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.