Isolated rat CNS mitochondria and cultured cortical astrocytes were examined for behavior indicative of a mitochondrial permeability transition (mPT). Exposure of isolated CNS mitochondria to elevated calcium or phosphate or both produced loss of absorbance indicative of mitochondrial swelling. The absorbance decreases were prevented by ADP and Mg2+ and reduced by cyclosporin A, dithiothreitol, and N-ethylmaleimide. Ruthenium red prevented calcium cycling-induced, but only attenuated phosphate-induced losses of absorbance. In cultured astrocytes permeabilized with digitonin or treated with the calcium ionophore, 4-bromo-A23187, elevations of external calcium altered mitochondrial morphology visualized with the dye, JC-1, from rod-like to rounded, swollen structures. Similar changes were observed in digitonin-permeabilized astrocytes exposed to phosphate VSports手机版. The incidence of calcium-induced changes in astrocyte mitochondria was prevented by Mg2+ and pretreatment with dithiothreitol and N-ethylmaleimide, and was reduced by cyclosporin A, ADP, and butacaine alone or in combinations. Ruthenium red and the Na+/Ca2+ exchange inhibitor CGP 37157 blocked calcium cycling and prevented mitochondrial shape changes in digitonin-treated, but not ionophore-treated astrocytes. Thus, the demonstrated induction conditions and pharmacological profile indicated the existence of an mPT in brain mitochondria. The mPT occurred consequent to activation of calcium cycling-dependent and -independent pathways. Induction of an mPT could contribute to neuronal injury following ischemia and reperfusion. .