It is increasingly recognised that mitochondria are potential targets to pharmacological and toxicological actions of membrane-active agents, including some 1,4-dihydropyridines derivatives (DHPs). The 5-acetyl(carbamoyl)-6-methylsulfanyl-1,4-dihydropyridine-5-carbonitriles (OSI-1146, OSI-3701, OSI-3761, and OSI-9642) is a new group of DHPs with minor differences on the molecular structure. It has also been shown that OSI-1146 displays cardiovascular, antioxidant, and antiradical activities, whereas OSI-3701 and OSI-3761 display hepatoprotective activity. Due to their protective properties, this group of DHPs may be potentially useful for the treatment of several pathological processes, including those associated with oxidative stress. However, the cellular targets for their pharmacological actions have not been investigated. The presented study, using isolated rat liver mitochondria was designed to investigate if mitochondria are a cellular target for the pharmacological and/or toxicological actions of these new group of DHPs. We studied the direct influence of these DHPs on rat liver mitochondrial function [bioenergetics, membrane permeability transition (MPT), and oxidative stress]. It was shown that OSI-1146, OSI-3761, and OSI-9642, in the concentration range of up to 200 microM, interfered with mitochondrial bioenergetics by affecting complexes I and II of the mitochondrial respiratory chain, the ATPase activity, and mitochondrial inner membrane permeability to protons. However, the effects of OSI-1146 were higher than those of OSI-3761 and OSI-9642. The remaining compound, OSI-3701, had no effect on the bioenergetic parameters tested. All the compounds increased the susceptibility of mitochondria to MPT, but, OSI-3701, not affecting the bioenergetic parameters, was the most potent. Moreover, all the compounds protected mitochondria against lipid peroxidation induced by the oxidant pair ADP/Fe(2+), but OSI-1146 was also the most potent. In conclusion, our results indicate that mitochondria are the potential intracellular targets for both protective and toxicological actions of the DHP compounds studied, suggesting that the potential use of these compounds as therapeutic agents should carefully consider their toxic effects to mitochondria.