Pea stem mitochondria, resuspended in a KCl medium (de-energized mitochondria), underwent a swelling, as a consequence of K+ entry, that was inhibited by ATP. This inhibition was partially restored by GTP and diazoxide (K+ATP channel openers). In addition, glyburide and 5-hydroxydecanate (K+ATP channel blockers) induced an inhibition of the GTP-stimulated swelling. Mitochondrial swelling was inhibited by H2O2, but stimulated by NO. The same type of responses was also obtained in succinate-energized mitochondria. When the succinate-dependent transmembrane electrical potential (deltapsi) had reached a steady state, the addition of KCl induced a dissipation that was inhibited by H2O2 and stimulated by NO. The latter stimulation was prevented by carboxy-PTIO, a NO scavenger. Phenylarsine oxide (a thiol oxidant) and NEM (a thiol blocker) stimulated the KCl-induced dissipation of deltapsi, while DTE prevented this effect in both cases. In addition, DTE transiently inhibited the NO-induced dissipation of deltapsi, but then it caused a more rapid collapse. These results, therefore, show that the plant mitochondrial K+ATP channel resembles that present in mammalian mitochondria and that it appears to be modulated by dithiol-disulfide interconversion, NO and H2O2. The aperture of this channel was linked to the partial rupture of the outer membrane. The latter effect led to a release of cytochrome c, thus suggesting that this release may be involved in the manifestation of programmed cell death.