The possible inhibition of ursolic acid (UA) on mitochondrial permeability transition (MPT) in mouse liver was investigated to identify the mechanisms underlying the hepatoprotective effect of UA. The effect of UA on liver MPT induced by Ca2+ was assessed by measuring changes in mitochondrial volume, mitochondrial membrane potential (MMP), release of matrix Ca2+, and transfer of cytochrome c (Cyt c) and apoptosis-inducing factor (AIF) from the intermembrane space to the cytoplasm. The results showed that obvious mitochondrial swelling, loss of MMP, and release of matrix Ca2+ occurred after the addition of 50 lM Ca2+. However, preincubation with 20, 50 or 100 lg ml 1 UA significantly blocked the above changes. Addition of 100lg ml1 UA inhibited on mitochondrial swelling by 73.2% after 5min, while the MMP dissipating and Ca2+ releasing were, respectively, suppressed by 59.3% and 54.1% after 3min. In addition, Western blot analysis showed Cyt c and AIF transferred from mitochondrial pellet to the supernatant after the addition of 50lM Ca2+, but the process was significantly inhibited by various concentrations of UA. The results suggest that the mechanisms underlying the hepatoprotection of UA may be related to its direct inhibitory action on MPT.