Retinoic acids, structurally related to vitamin A, inhibit the in vitro proliferation of different types of normal and neoplastic cells. The effects of all-trans, 9-cis, and 13-cis retinoic acids were tested on mitochondria isolated from rat liver. All the compounds were able to induce the membrane permeability transition observed as swelling and decrease in membrane potential, but 13-cis retinoic acid appeared to be the most effective. The latter was also shown to stimulate the release of cytochrome c from mitochondria, suggesting a potential target of retinoids in the induction of cell apoptosis. Interestingly, EGTA and cyclosporin A, which strongly inhibit the permeability transition induced by 13-cis retinoic acid, were without effect on the release of cytochrome c from the mitochondrial intermembrane space.