Our previous investigations demonstrated that neurons in the area between the subiculum and the medial CA1 region and another area between the lateral CA1 and the CA3 region of the hippocampus are very vulnerable to cerebral ischemia in gerbils, where irreversible damages have been observed occur as early as 4-5 min after unilateral or bilateral common carotid artery occlusion. The present study was aimed to characterize these areas anatomically by using the immunohistochemical and zinc histochemical as well as Golgi silver impregnation methods. Our results indicated that these two areas which are topographically apart on the coronal section actually had a common origin in the rostral part of the hippocampus and that they were separated by insertion of the Ca1 neurons between them in the more caudal part of the hippocampus. The distribution pattern of mossy fibers indicated that these areas belonged to the CA2 region. The double immunohistochemical and zinc histochemical procedure confirmed that these areas developed ischemic lesions promptly even without reperfusion but that the CA1 region did not develop similar lesions until after reperfusion for 12-24 h if the ischemic period was brief. While the reason for the observed susceptibility of the CA2 region is not certain at the present time, it is important to distinguish the ischemic lesions in the CA2 region from those in the CA1 and CA3 regions.