Brain mitochondria purified by phase partition showed a higher rate of 14C-leucine incorporation into proteins with an endogenous source of ATP than with an exogenous ATP-generating system. Under the former conditions the presence of atractyloside increased the 14C-leucine incorporation into proteins. The effects of different valinomycin concentrations plus attractyloside on intramitochondrial ATP levels and 14C-leucine incorporation into proteins have been studied. The results indicate that the protein synthesis in brain mitochondria is dependent on the intramitochondrial ATP concentration.