We investigated the temporal profile of heat shock protein 70 induction in the rat hippocampus using immunohistochemistry to clarify the mechanism of ischemic tolerance following preconditioning with sublethal ischemia. Although a 6-min period of forebrain ischemia produced severe neuronal damage to the hippocampal CA1 subfield, preconditioning with 3 min of ischemia followed by three days of reperfusion protected against the CA1 neuronal damage after 6 min of ischemia. Immunohistochemical staining against heat shock protein 70 showed that the protein is induced in CA1 pyramidal cells one, three and seven days after 3 min of ischemia, the immunostaining being most intense after three days. Heat shock protein synthesis was observed in CA1, CA3 and dentate hilar neurons one and three days after 6 min of ischemia, both with and without preconditioning. In addition, the heat shock protein was stained in the CA1 2 h and seven days after 6 min of ischemia with preconditioning, but the intensity of staining was relatively weak at these time points. The results suggest that stress response induced by sublethal ischemia protects against ischemic neuronal damage, and that the induced stress response, including heat shock protein 70 synthesis during and immediately after the second ischemic episode, is correlated with the protection because late induction of the heat shock protein did not prevent neuronal death.