Rabbit heart mitochondria isolated with or without rotenone (M + R and M -R, respectively) were studied. It was shown with the use of pHmetry that the Vmax of oxidative phosphorylation decreased during ischemia (autolysis for 0.5 h at 37 degrees C) in both preparations of the mitochondria to the same extent. The Km for ADP was unchanged in M - R, but rose significantly in M + R. Carnitine reduced the ischemia -increased Km value. The data suggest the inhibition of adenine nucleotide translocase by longchain acyl-Co A, whose level was found to be elevated by 52% in M + R during ischemia.