The effects of auranofin, chloro(triethylphosphine)gold(I) (TEPAu), and aurothiomalate on mitochondrial respiration, pyridine nucleotide redox state, membrane permeability properties, and redox enzymes activities were compared. The three gold(I) derivatives, in the submicromolar range, were extremely potent inhibitors of thioredoxin reductase and stimulators of the mitochondrial membrane permeability transition (MPT). Auranofin appeared as the most effective one. In the micromolar range, it inhibited respiratory chain and glutathione peroxidase activity only slightly if not at all. TEPAu and aurothiomalate exhibited effects similar to auranofin, although TEPAu showed a moderate inhibition on respiration. Aurothiomalate inhibited glutathione peroxidase at concentrations where auranofin and TEPAu were without effect. Under nonswelling conditions, the presence of auranofin and aurothiomalate did not alter the redox properties of the mitochondrial pyridine nucleotides indicating that membrane permeability transition occurred independently of the preliminary oxidation of pyridine nucleotides. Under the same experimental conditions, TEPAu showed a moderate stimulation of pyridine nucleotides oxidation. Mitochondrial total thiol groups, in the presence of the gold(I) derivatives, slightly decreased, indicating the occurrence of an oxidative trend. Concomitantly with MPT, gold(I) compounds determined the release of cytochrome c that, however, occurred also in the presence of cyclosporin A and, partially, of EGTA, indicating its independence of MPT. It is concluded that the specific inhibition of thioredoxin reductase by gold(I) compounds may be the determinant of MPT and the release of cytochrome c.