The mitochondrial permeability transition (MPT) resulting from calcium-induced opening of cyclosporin A (CsA)-sensitive megachannels, leading to deenergisation of mitochondria and release of pro-apoptotic cytochrome c, has been implicated in the pathomechanism of excitotoxic neurodegeneration. The aim of this work was to test neuroprotective potential of CsA in the model of N-methyl-D-aspartate-(NMDA)-induced excitotoxicity in vivo, and to verify utility of microdialysis of the rabbit hippocampus in vivo for these mechanistic studies. In vitro experiments demonstrated that the early rapid phase of Ca(2+)-induced swelling of isolated brain mitochondria, and of accompanying cytochrome c release, was strongly inhibited by 0.5 microM CsA. In the in vivo experiments 1 mM NMDA was applied for 20 min to the hippocampus in a control, or 5 microM CsA-containing dialysis medium via transhippocampal microdialysis probes, and changes in extracellular Ca2+ concentration and in NO release were monitored. Application of NMDA induced a prolonged decrease in the extracellular concentration of Ca2+, reflecting influx of Ca2+ to stimulated neurones. CsA only slightly enhanced this effect. NMDA induced also release of NO to the dialysis medium. Morphological examination 30 min after NMDA application visualised swelling of dendritic mitochondria and cisternae of endoplasmatic reticulum of pyramidal neurones in the CA1 sector of the hippocampus in the vicinity of microdialysis probes. CsA prevented mitochondrial swelling. After 24 h degeneration of the CA1 pyramidal neurones close to a microdialysis probes was observed, which was partially prevented in CsA-treated rabbits. These results indicate that the mechanism of CsA nuroprotection may be at least in part ascribed to prevention of MPT. Microdialysis of the rabbit hippocampus combined with NMDA excitotoxicity appeared to be useful in mechanistic studies of CsA neuroprotection.