1. Inhibition of either mitochondrial transcription or translation in Neurospora crassa results in the rapid production of the cyanide-insensitive pathway of mitochondrial respiration. Protein synthesis on cytoplasmic ribosomes is required for the appearance of cyanide-insensitive respiration in the culture. 2. Removal of the inhibition of transcription of translation results in a rapid return to cyanide-sensitive respiration. Additional protein synthesis in the cytoplasm is required for the loss of cyanide-insensitive respiratory activity while additional mitochondrial protein synthesis has no effect. 3. These studies indicate that a mitochondrial gene product is involved in a negative manner in the regulation of cyanide-insensitive respiration. When the mitochondrial product is present, the pathway is not expressed, when the mitochondrial product is absent, the pathway is expressed. 4. Studies with forced heterokaryons formed from respiratory-deficient mutants having cyanide-insensitive respiration and respiratory-competent auxotrophs indicate that the site of action of the mitochondrial gene product is external to the mitochondrion.