The organic nitrate esters, glycerol trinitrate (GTN), mannitol hexanitrate (MHN) and erythritol tetranitrate (ETN) were tested for effects on the oxygen consumption of rat liver mitochondria. Both MHN and GTN are capable of reducing the respiratory control ratio to 1.0 [rate of O2 consumption with ADP (state 3)/rate without ADP (state 4)] However, this requires 1 mM GTN as compared to 0.1 mM MHN, and the actions are effected through different mechanisms. With the NAD-linked substrate, 3-hydroxybutyrate, GTN inhibits the state 3 rate, whereas MHN markedly increases the state 4 rate. GTN, in concentrations that inhibit 3-hydroxybutyrate oxidation, has little or no effect on respiration with succinate as substrate. MHN, on the other hand, accelerates the state 4 rate with succinate just as it does with 3-hydroxybutyrate. Therefore, the action of GTN appears to be selective for the NADH dehydrogenase region of the electron transport chain. In contrast to the generalized uncoupling activity of MHN, the predominant effect of GTN appears to be inhibition of electron transport. Since DNP relieves this inhibition to a large degree, a considerable part of it must be indirect, by way of an inhibition in energy transfer reactions, as with oligomycin or atractyloside. There also appears to be a small direct inhibition of electron transport by GTN and MHN at low concentrations. This inhibition increases with higher concentrations. Erythritol tetranitrate appears to be intermediate between MHN and GTN in the effects produced. It is suggested that lipid solubility of the organic nitrate esters, location of sulfhydryl groups critical for electron transfer and energy conservation, and reactivity of the organic nitrate esters with sulfhydryl groups determine the selective effects observed.