The effects of lanthanum on heat production of mitochondria isolated from Wistar rat liver were investigated with microcalorimetry; simultaneously, the effects on mitochondrial swelling and membrane potential (Δψ) were determined by spectroscopic methods. La(3+) showed only inhibitory action on mitochondrial energy turnover with IC50 being 55.8 μmol L(-1). In the spectroscopic experiments, La(3+), like Ca(2+), induced rat liver mitochondrial swelling and decreased membrane potential (Δψ), which was inhibited by the specific permeability transition inhibitor, cyclosporine A (CsA). The induction ability of La(3+) was stronger than that of Ca(2+). These results demonstrated that La(3+) had some biotoxicity effect on mitochondria; the effects of La(3+) and Ca(2+) on rat liver mitochondrial membrane permeability transition (MPT) are different, and La represents toxic action rather than Ca analogy.