Excitotoxicity is generally studied in dissociated neurons, cultured hippocampal slices, or intact animals. However, the requirements of dissociated neurons or cultured slices to use prenatal or juvenile rats seriously limit the advantages of these systems, whereas the complexity of intact animals prevents detailed molecular investigations. In the present experiments, we studied developmental changes in NMDA neurotoxicity in acute hippocampal slices with lactate dehydrogenase (LDH) release in medium, propidium iodide (PI) uptake, and Nissl staining as markers of cell damage. Calpain-mediated spectrin degradation was used to test calpain involvement in NMDA neurotoxicity. NMDA treatment produced increased LDH release, PI uptake, and spectrin degradation in slices from juvenile rats but not adult rats. NMDA-induced changes in slices from young rats were blocked completely by the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801) and by the antagonists of NR2B receptor ifenprodil and R-(R, S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol and were partly blocked by calpain inhibitor III but were not affected by the NR2A-specific antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid. NMDA-induced changes in Nissl staining were also different in slices from young and adult rats and blocked by NR2B but not NR2A antagonists. In contrast to NMDA treatment, oxygen/glucose deprivation (OGD) induced neurotoxicity in slices from both young and adult rats, although OGD-induced toxicity was attenuated by MK-801 only in slices from young rats. Our results are consistent with the idea that NMDA-mediated toxicity is caused by activation of NR2B- but not NR2A-containing NMDA receptors leading to calpain activation and that developmental changes in NMDA toxicity reflect developmental changes in NMDA receptor subunit composition.