In contrast to the thoroughness with which the respiratory enzymes of such tissues as heart muscle and liver have been studied, information concerning these enzymes in the adrenal gland is scant. Such information as is available suggests that an unusual pattern for the cytochromes exists in this tissue. Cohen and Elvehjem (2) commented on the "peculiar cytochrome spectrum"of the adrenal medulla of the cow. They concluded that a very strong cytochrome c component was present, and that cytochrome a and 6 were absent. Huszák (3) noted that the adrenal cortex possessed a nearly normal cytochrome pattern. In the medulla he could observe no cytochrome a or c bands, but reported a strong band in the region of 559 rnp which he attributed to cytochrome 5. Since Huszák could demonstrate no cytochrome oxidase activity in the medulla, he felt that oxidation of substrates proceeded by way of a peroxidative mechanism. Tsou (4), on the other hand, was able to demonstrate cytochrome oxidase activity in the medulla, but little if any cytochrome c. He observed the presence of a strong band at 561 millimicron which was referred to as cytochrome b. In view of these divergent conclusions and in the light of newer methods and knowledge, it seemed worth-while to reinvestigate some of the oxidative pathways in the adrenal gland. The present study deals with the adrenal medulla, and an accompanying paper deals with the cortex (5). It will be shown here that the adrenal medulla contains the regular mitochondrial cytochrome system. In addition, this tissue contains a hemochromogen which is apparently microsomal in origin and is abundantly present in the epinephrine-containing granules. The presence in the medulla of the enzymes succinate dehydrogenase,reduced di- and triphosphopyridine nucleotide-cytochrome c reductase,and transhydrogenase is also demonstrated.