Spectroscopic observations on beef adrenal cortex indicate the presence of cytochrome c, c1, b, and a+a3. The absorption at 563 m[mu], presumably caused by cytochrome b, dominates the picture. The presence of cytochrome c1 has not been demonstrated directly, but is inferred from the location and height of an absorption band at 552 to 553 m[mu] and the content of cytochrome c as determined by extraction. The cytochrome c content of the cortex so determined is 3 times that of the medulla but only 1/7 that of heart muscle. A microsomal cytochrome of the type so abundantly present in the medulla could not be demonstrated in the cortex. However, proof of its complete absence is made difficult by the fact that it has not been possible to obtain by differential centrifugation techniques a microsomal fraction free of mitochondria. This is apparently because of the marked pleomorphism of the mitochondria of the cortex. The succinate dehydrogenase, reduced diphospho-and triphosphophyridine nucleotide-cytochrome c reductase and transhydrogenase activity in various tissue fractions have been studied. The cortex is extremely rich in transhydrogenase activity in comparison to values reported in the literature for this enzyme in other mammalian tissues. This activity is discussed in relation to the high reduced triphosphopyridine nucleotide-generating capacity of the cortex and the synthesis of steroids by this tissue.