We generated defined neuronal loss in hippocampus of genetically identical mice by pilocarpine injections and studied the impact of these seizures on the performance of mice in spatial learning and memory. The numbers of TUNEL-positive degenerating cells paralleled the severity of the seizures. When compared to the numbers found for not-seizured control mice, mild, moderate, and severe seizures produced significant increases in TUNEL-positive neurons in CA1 and CA3 regions by 19, 25, and 63%, respectively. Water maze learning was abolished after the severe seizures. However, spatial learning was normal after mild or moderate seizures. Therefore, there was no linear correlation between the impairment of learning and memory performance with the number of degenerating neurons in hippocampus. Our data suggest that normal spatial learning and memory can be achieved without the full number of hippocampal pyramidal neurons in partially lesioned hippocampus.