The in vitro effects of iron (III)-gluconate complex on the production of malondialdehyde and on the Ca 2+ transport in isolated rat liver mitochondria were studied. A correlation between the concentration of iron added and the formation of malondialdehyde was found. The enhancement by iron of lipid peroxidative process in the mitochondrial membrane brought about the induction of Ca 2+ release from mitochondria. Experimental evidence based on the membrane potential pattern of mitochondria pre-loaded with a low pulse of Ca 2+ suggested that Ca 2+ efflux was not due to a nonspecific increase in the inner membrane permeability, i.e. to a collapse of membrane potential, but rather to the activation of an apparently selective pathway for Ca 2+ release.