Cytochrome c was detected by immunoblotting in the cytosolic fraction 3 h after 5-min ischemia in the non-ischemia-tolerant CA1 region in which about 96% of neurons had developed delayed neuronal death, while less cytosolic cytochrome c was detected in the ischemia-tolerance-induced CA1 region where many more neurons survived. In the immunohistochemical study using anti-non-native cytochrome c monoclonal antibody, immunoreactivity was observed throughout the cytoplasm in the non-ischemia-tolerant CA1 neurons, but not in the normal and ischemia-tolerant CA1 neurons. Then we determined whether Bcl-2, Bax, Bcl-xL and Bcl-xS, which regulate the release of cytochrome c from mitochondria, were altered in the ischemia-tolerant CA1 region. Bcl-2 and Bax were up-regulated in the ischemia-tolerant group, but Bcl-xL and Bcl-xS showed no apparent difference in their expression. These results suggest that cytochrome c release is prevented in CA1 neurons in gerbils in which ischemia-tolerance had been induced and that the altered ratio of Bcl-2 to Bax may play a part in this mechanism.