The objective of this work was to evaluate whether changes in respiratory status of isolated brown-fat mitochondria influence synthesis and degradation of proteins within the organelles. Mitochondrial protein synthesis is subject to regulation, as a 24-h fast in mice reduced [35S]methionine incorporation without affecting the stability of the newly synthesized proteins. Proteins synthesized in isolated mitochondria were labile and degraded in a process stimulated by ATP. ATP hydrolysis was required within the organelle, as ATP-stimulated protein breakdown was inhibited by atractyloside, an inhibitor of adenine nucleotide transport, and by arsenate and vanadate, which inhibit ATPases. Additions of ATP and ADP were equally effective at reducing mitochondrial oxygen consumption. However, ATP added exogenously was better at supporting protein synthesis and degradation than ATP generated by oxidative phosphorylation when mitochondria were incubated with ADP, substrates, and Pi. GDP also reduced oxygen consumption and stimulated degradation of mitochondrial translation products. Addition of fatty acids in the presence or absence of carnitine-CoA increased mitochondrial respiration but had no effect on protein synthesis or degradation. Addition of carbonyl cyanide m-chlorophenylhydrazone (mCCP) had no effect on ATP-stimulated protein degradation. These results indicate that synthesis and stability of mitochondrial translation products are not significantly influenced by changes in the activity of the uncoupling protein brought about by additions of adenine nucleotides and fatty acids.