Publisher Summary Extrinsic and intrinsic fluorescence probes have been used to study conformational changes and nucleotide binding sites in the mitochondrial adenosine diphosphate (ADP)/ adenosine triphosphate (ATP) carrier protein. This carrier catalyzes the exchange between external and internal ADP and ATP across the inner membrane of mitochondria. The extrinsic fluorescence probes to be described below include representative fluorescent analogs of ADP or ATP—namely, naphthoyl-ADP and naphthoyl-ATP, two nontransportable nucleotides which however bind with high affinity to the ADP/ATP carrier, and formycin triphosphate, a transportable nucleotide. The fluorescence changes monitored by extrinsic and intrinsic fluorescent probes have led to the conclusion that the ADP/ATP carrier can adopt two conformations probably associated with the transport process. The CATR conformation is recognized by carboxyatractyloside (CATR) and its derivative atractyloside (ATR), two inhibitors which attack the ADP/ATP carrier in mitochondria from the outside, whereas the BA conformation is recognized by bongkrekic acid (BA), an inhibitor which attacks the carrier from the inside, after penetration into the mitochondrial matrix.