It is proposed to determine the precise site of binding of 2-azido-4-nitro-phenol to mitochondria of brown adipose and liver mitochondria from the golden hamster which has been cold acclimated at 5 degrees C., following flash photolysis to covalently bond the uncoupler to the peptides of the mitochondrial inner membrane. It is also proposed to characterize the metabolites of 9,10-dibromopalmitic acid, a non-oxidizable fatty acid analog, and to relate its uncoupling action to its possible alterations by the isolated mitochondrial suspensions. The results of these separate lines of investigation will be combined to determine how the azidonitrophenol and the brominated fatty acid compete for mitochondrial binding sites.