We investigated the effects of ischemia on the kinetics and control of mitochondria isolated from normal and ischemic heart. The dependence of the respiratory chain, phosphorylation system and proton leak on the mitochondrial membrane potential were measured in mitochondria from hearts after 0, 30 min and 45 min of in vitro ischemia. Data showed that during the development of ischemia from the reversible (30 min) to the irreversible (45 min) phase, a progressive decrease in activity of the respiratory chain occurs. At the same time an increase in proton leak across the mitochondrial inner membrane was observed. Phosphorylation is inhibited but seems to be less affected by ischemia than respiratory chain or proton leak. Control coefficients of the 3 blocks of reactions over respiration rate were determined in different respiratory states between state 4 and state 3. Ischemia caused the control exerted by the proton leak to increase in state 3 and the intermediate state and caused the control by the phosphorylation system to decrease in the intermediate state. Taken together, these results indicate that the main effects of ischemia on mitochondrial respiration are an inhibition of the respiratory chain and an increase of the proton leak.