The presence of cytochrome oxidase in the nuclear material from chicken erythrocytes and rat-liver cells has been demonstrated with the aid of p-phenylenediamine as the agent for reducing cytochrome c. The three reagents p-phenylenediamine, ascorbate, and hydroquinone are effective in the estimation of cytochrome oxidase in mitochondrial preparations, but only the first mentioned agent can effect the reduction of endogenous cytochrome c. The addition of cytochrome c will increase the oxidase activity of the liver-cell mitochondria but not of the nuclear fraction from the erythrocytes or the liver cells, presumably because of the impermeability of the nuclear membranes to added cytochrome c.