1. We investigated the effects of nimesulide, a recently developed non-steroidal anti-inflammatory drug, and of a metabolite resulting from reduction of the nitro group to an amine derivative, on succinate-energized isolated rat liver mitochondria incubated in the absence or presence of 20 microM Ca(2+), 1 microM cyclosporin A (CsA) or 5 microM ruthenium red. 2. Nimesulide uncoupled mitochondria through a protonophoretic mechanism and oxidized mitochondrial NAD(P)H, both effects presenting an EC(50) of approximately 5 microM. 3. Within the same concentration range nimesulide induced mitochondrial Ca(2+) efflux in a partly ruthenium red-sensitive manner, and induced mitochondrial permeability transition (MPT) when ruthenium red was added after Ca(2+) uptake by mitochondria. Nimesulide induced MPT even in de-energized mitochondria incubated with 0.5 mM Ca(2+). 4. Both Ca(2+) efflux and MPT were prevented to a similar extent by CsA, Mg(2+), ADP, ATP and butylhydroxytoluene, whereas dithiothreitol and N-ethylmaleimide, which markedly prevented MPT, had only a partial or no effect on Ca(2+) efflux, respectively. 5. The reduction of the nitro group of nimesulide to an amine derivative completely suppressed the above mitochondrial responses, indicating that the nitro group determines both the protonophoretic and NAD(P)H oxidant properties of the drug. 6. The nimesulide reduction product demonstrated a partial protective effect against accumulation of reactive oxygen species derived from mitochondria under conditions of oxidative stress like those resulting from the presence of t-butyl hydroperoxide. 7. The main conclusion is that nimesulide, on account of its nitro group, acts as a potent protonophoretic uncoupler and NAD(P)H oxidant on isolated rat liver mitochondria, inducing Ca(2+) efflux or MPT within a concentration range which can be reached in vivo, thus presenting the potential ability to interfere with the energy and Ca(2+) homeostasis in the liver cell.