Brown‐adipose‐tissue mitochondria possess an energy‐dissipating ion uniport which is inhibited by purine nucleotides. The regulatory nucleotides bind to a high‐affinity site on the outer face of the inner membrane which is independent of the adenine nucleotide translocator. A direct correlation between affinity for the regulatory site and ability to inhibit the ion uniport is demonstrated for a number of nucleotide analogues. 8‐Azido‐adenosine 5′‐triphosphate, a photoaffinity label, also competes with GDP for the binding site and induces respiratory control. 8‐Azido‐adenosine [γ‐³²P]triphosphate was prepared and covalently bound to hamster brown‐adipose‐tissue mitochondria by near‐ultraviolet irradiation. Two major radioactive bands were identified of apparent molecular weight 30 000 and 32 000, representing 6% and 10% of the inner membrane protein respectively. Selective labelling enabled the 30 000‐M_r protein to be identified as the carboxyatractylate binding component of the adenine‐nucleotide translocator and the 32 000‐M_r protein to be identified as the regulatory site of the energy‐dissipating ion uniport. The levels of the 32 000‐M_r protein in the inner membrane of guinea‐pig brown‐adipose‐tissue mitochondria correlate with the degree of thermogenic adaptation of the animal.