Oxidative stress causes Ca(2+)-loaded mitochondria to release Ca2+. The mechanism of this efflux is unclear, but it appears to be associated with the opening of a pore in the mitochondrial inner membrane. Pore opening depolarizes the mitochondria, letting solutes enter the mitochondrial matrix, causing swelling. Cyclosporin A (CsA) prevents opening of this pore. The neurotoxin 6-hydroxydopamine (6HD) autoxidizes, producing free radicals, which cause oxidative stress. In this paper it is shown that 6HD-induced efflux from Ca(2+)-loaded mitochondria was prevented by CsA. The 6HD-induced Ca2+ efflux was not accompanied by mitochondrial swelling, depolarization of the mitochondrial inner membrane or movement of radiolabelled sucrose into the mitochondrial matrix. In agreement with others [Schlegel, Schweizer and Richter (1992) Biochem. J. 285, 65-69], these findings suggest that the mitochondrial pore remained closed during pro-oxidant-induced Ca2+ efflux. However, the implication that CsA blocks pro-oxidant-induced Ca2+ efflux by some mechanism other than inactivating the mitochondrial pore, suggests that the interaction of CsA with mitochondria may be more complex than is currently supposed.