Double aldehyde fixed carotid bodies and small pieces of vagus nerve of cats were incubated in 3 mM copper sulfate and 0.5 mM potassium ferricyanide in 0.05 M acetate buffer (pH 5.6) for 30 minutes at room temperature. Several modifications of this procedure were also attempted. Tissues were then postosmicated with 2% unbuffered osmium tetroxide and heated to 50-55 C for ten minutes. Under the electron microscope carotid body cells exhibited fine osmium deposits within cisternae of endoplasmic reticulum, saccula and vesicles of Golgi complex, and cristae of mitochondria. Intense osmium precipitation was also noted in the mitochondria of nerve endings. In addition, much more intense, more conspicuous and more localized reaction was noted in the intraperiod lines of the myelin sheath of nerves. Deposits here were rod-shaped, displaying considerable variation in length. These results are discussed in the light of previous findings on osmium deposits in various tissues. It was concluded that the osmium reaction is unspecific, and that histochemical methods employing hot osmium tetroxide to amplify enzymatic activities may therefore not be reliable.