Injection of N-methyl-D-aspartate (NMDA) or kainate in the striatum of 7-day-old rats induced massive cell loss in the ipsilateral striatum, hippocampus and inner cortical layers. In order to examine whether apoptosis contributes to cell death in this model of excitotoxic injury we examined the progression of internucleosomal DNA fragmentation and changes in cellular ultrastructure. Agarose gel electrophoresis of DNA extracted from the ipsilateral striatum, cerebral cortex and hippocampus clearly showed breakdown of DNA into oligonucleosome-sized fragments, indicative of apoptosis, 12 h post-NMDA injection. In addition, an increase between 12 and 24 h was observed as well as a continuous presence 5 days later. Kainate induced a similar time course of oligonucleosomal DNA fragmentation, but the intensity of the ethidium bromide stained bands was less compared with that observed for NMDA. DNA fragmentation was not detected in animals intrastriatally injected with Tris-HCl or in animals treated with MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohept-5,10-imine hydrogen maleate, 1 mg/kg] 30 min after NMDA injection. MK-801 had no effect on DNA fragmentation induced by kainate. In addition to agarose gel electrophoresis, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labelling (TUNEL) was used for detection of DNA fragmentation in sections. A gradual increase in the density of both apoptotic and non-apoptotic TUNEL nuclei was found in the anterior cingulate (ACC) and retrosplenial (RSC) areas of the cortex, the striatum, and the CA1 area and dentate gyrus of the hippocampus over the first 24 h post-NMDA or kainate injection. In the contralateral hemisphere hardly any TUNEL nuclei were present and their density was comparable with that in animals injected with vehicle only. In the ipsilateral mammillary nucleus (MN), which showed no signs of acute cell swelling after intrastriatal injection with NMDA, internucleosomal DNA fragmentation was found 24 and 48 h after intrastriatal NMDA injection. Here, the density of TUNEL cells with apoptotic morphology was high at 12 and 24 h post-NMDA injection but returned to control levels by 5 days. Electron microscopy showed cells with a clearly apoptotic morphology in the ACC and RSC and in the MN 24 h after NMDA injection. In the CA1 area of the hippocampus a necrotic, rather than an apoptotic, ultrastructure prevailed, indicating that the TUNEL method stained both apoptotic and necrotic cells.(ABSTRACT TRUNCATED AT 400 WORDS)