The effects of daphnetoxin on isolated rat liver mitochondria and freshly isolated rat hepatocytes were investigated. Daphnetoxin (in the microM range) increased mitochondrial state 4 respiration and decreased both state 3 and FCCP-uncoupled respiration. The transmembrane potential was strongly depressed by daphnetoxin in a concentration-dependent manner. The protonophoric activity of daphnetoxin was evidenced by the induction of mitochondrial swelling in hyposmotic K+ acetate medium in the presence of valinomycin. In isolated hepatocytes, daphnetoxin decreases intracellular ATP and simultaneously increases ADP and AMP concentrations. The addition of uncoupling concentrations of daphnetoxin to Ca2+-loaded mitochondria treated with Ruthenium Red results in non-specific membrane permeabilization, as evidenced by mitochondrial swelling in isosmotic sucrose medium. Mitochondrial swelling in the presence of Ca2+ was prevented by cyclosporine A and was drastically inhibited by catalase and dithiothreitol, indicating the participation of mitochondrial generated reactive oxygen species in this process. From this study we can conclude that the bioenergetic lesion promoted by daphnetoxin seems to be sufficient to explain the lethal hapatocyte injury.