Nitrite, which is the major stable degradation product of nitric oxide, exists in all tissues capable of nitric oxide synthesis from L-arginine. The present study provides experimental evidence that nitrite in contact with respiring mitochondria accepts reducing equivalents from the ubiquinone cycle of the respiratory chain. Univalent reduction of nitrite was totally inhibited by myxothiazol. We therefore conclude on the involvement of redox cycling that ubisemiquinone is associated with the bc1 complex. Recycling of nitric oxide degradation products via these electron carriers may become a threat to energy-linked respiration since nitric oxide in direct contact with mitochondria was shown to slow the energy-linked respiration down and to trigger a mitochondrial source for superoxide radicals. Until now, the existence of nitrite reductase activity was only demonstrated in plants and bacteria. In addition, the present observation elucidates the existence of a nitric oxide synthase-independent nitric oxide source.