The proton conductance of the inner membrane of hamster brown adipose tissue mitochondria can be regulated in vitro by exogenous purine nucleotides, which bind to a component on the outer face of the inner membrane. This unique mechanism has been proposed to represent the molecular site of non-shivering thermogenesis in this tissue. Using a photo-affinity analogue of ATP, we have identified the nucleotide binding component as a protein of 32,000 daltons.