We have shown earlier that microcystin-LR (MLR), a specific hepatotoxin, induced onset of mitochondrial permeability transition (MPT) and apoptosis in rat hepatocytes. Here we attempted to investigate the role of mitochondrial Ca2+ in MLR-induced onset of MPT and cell death. Using confocal microscopy, we found that MLR caused an early surge of mitochondrial Ca2+ prior to the onset of MPT and cell death. Pretreatment with 1,2-bis(O-aminophenoxyl)ethane-N,N,N ' ,N ' -tetracetic acid tetra(acetoxymethyl)ester (an intracellular Ca2+ chelator) or ruthenium red (an inhibitor of mitochondrial Ca2+ uniporter) prevented the early mitochondrial Ca2+ surge and attenuated the subsequent onset of MPT and cell death. On the other hand, a mitochondrial uncoupler, CCCP, rapidly disrupted the mitochondrial membrane potential and also prevented the mitochondrial Ca2+ surge, onset of MPT, and cell death. We thus conclude that mitochondrial Ca2+ plays an important role in the onset of MPT and cell death in MLR-treated rat hepatocytes.