1. Investigation of a number of reactions involving both internal and externally added adenine nucleotides of isolated liver mitochondria has revealed that atractylate and oligomycin differ markedly in the site of their inhibitory action. 2. Both atractylate and oligomycin inhibited the respiratory-chain-level phosphorylation of added ADP. Neither compound inhibited the substrate-level phosphorylation of internal (endogenous) ADP or the respiration-dependent accumulation of bivalent metal ions (Ca(2+), Sr(2+) or Mn(2+)). 3. Atractylate, but not oligomycin, inhibited the substrate-level phosphorylation of externally added ADP, the ATP- and carnitine-dependent reduction of nicotinamide nucleotide by palmitate and the ATP-induced activation of succinate oxidation. 4. Oligomycin, but not atractylate, inhibited the respiratory-chain-linked phosphorylation of internal ADP, and the dephosphorylation of internal ATP that occurred on the addition of antimycin. 5. The enhancement of arsenate-stimulated respiration by ADP was prevented by atractylate added either before or after the ADP. Oligomycin abolished both the arsenate and ADP stimulation. 6. It is suggested that atractylate prevents the passage of adenine nucleotides across the mitochondrial membrane, whereas oligomycin interferes with the formation of a ;high-energy' phosphorylated intermediate.