Excessive activation of excitatory amino acid receptors has been implicated in neuronal death in a number of central nervous system insults. We have here investigated, the time course and mechanisms of kainate (KA)- induced neuronal death in immature organotypic hippocampal slice cultures (OHCs) using Fluoro-Jade B (FJB) staining as a marker of cell death, and immunoblotting, immunocytochemistry, and electron microscopy as methods to clarify the mechanisms. After 6 KA treatment (5 microM), no significant neuronal death was detected in any hippocampal subregion, whereas the treatment of 12, 24, and 48 h resulted in neuronal death in the CA3 regions, but not in CA1. The 48 h resting period in normal medium after KA-treatment did not rescue the cells but further increased the number of dead neurons in CA3 as compared to the corresponding acute phase. In Western blotting, the expression levels of the active, 17 kDa form of caspase-3, and the 84-85 kDa cleaved fragment of poly(ADP ribose)polymerase (PARP) were not altered from the control levels. Moreover, no active caspase-3 labelled cells were detected in immunocytochemical study 24 h after KA treatment either in the acute or resting groups. Electron microscopy showed non-apoptotic injury in the CA3a/b pyramidal neurons in KA-treated slices. Our results suggest that KA-induced neuronal death in immature OHCs is a strictly region-specific, irreversible, necrotic process.