Mitochondria isolated from rat livers ischemic at 24 ° for up to 13 hr exhibited nearly normal capacity to phosphorylate added ADP ( ADP 0 ratios) in the presence of bovine serum albumin (BSA). Respiratory control ratios (rate with ADP/rate without ADP) remained nearly normal for the first 3 hr, then declined somewhat. However, even at 13 hr the ratios were still 2 to 3 compared to normal values of 4 to 6. The decline of respiratory control was due primarily to a decline in the State 3 respiratory rate (i.e., with ADP and substrate). The demonstration of relatively normal oxidative phosphorylation in mitochondria isolated from livers after prolonged periods of ischemia at 24 ° is dependent on the use of BSA in the isolation medium and/or in the test medium. In the complete absence of BSA, or at 37 ° in the presence of BSA, the respiratory control is lost rapidly. These data are compatible with release of fatty acids in the ischemic cell being responsible for the apparent damage to mitochondria. Electron microscopic studies show very slow changes with time in mitochondria in ischemic liver cells, greater changes in the isolated mitochondria. During the isolation procedures and in the test medium, binding by BSA can remove the fatty acids and their uncoupling and inhibitory effects, revealing relatively intact enzymes of oxidative phosphorylation.