PURPOSE: Reorganization of mossy fiber terminals in the supragranular layer of the dentate has been found in hippocampi of human epileptics and animal models by Timm staining. Many studies have provided evidence that mossy fiber sprouting is strongly associated with neuronal loss. But the question of whether cell loss is necessary for stimulation of mossy fiber sprouting is remained to be answered. In this present study, we evaluated whether hippocampal mossy fiber sprouting is induced in damaged hippocampus of the rats exposed to hypoxic-ischemic insults in juvenile and adult period. METHODS: At ages of 4-5 weeks and 2 months, the experimental rats were received procedure of right carotid artery unilateral ligation under anesthesia. After 3 hours of the recovery period, they were placed in an airtight 2000ml chamber and exposed to a 8% oxygen-92% nitrogen mixture delivered at 5 liter/min for 90 minutes (juvenile) and 50 minutes (adult). After the recovery period, The animals were returned to cages and housed with controls. 2 weeks later, rats of the control and hypoxic-ischemia group were anesthetised and then perfused with sodium sulfide solusion and fixed. 40micrometer (for Timm stain) and 5micrometer (for H & E stain) coronary brain sections were obtained, stained with Timm method and H < E stain for the observation of the neuronal loss and supragranular Timm granules in the hippocampi. RESULTS: Light microscopic examination of the brains from hypoxic-ischemic animals demonstrated ischemic changes of variable degrees in the hippocampal hilar and pyramidal cell layers. No supragranular mossy fiber sprouting were found in hippocampi of juvenile and adult rats with hypoxic-ischemic damages. CONCLUSION: These results implicated that hippocampal mossy fiber sprouting is not induced in the experimental hypoxic-ischemic encephalopathy of juvenile and adult rats, although cellular loss is found in hippocampus. Neuronal loss might be not necessary for the development of mossy fiber sprouting.