Cytochrome c concentrations were determined in tissues of the rat and the guinea pig, two mammalian species of contrasting developmental pattern. The relatively mature newborn guinea pig had approached adult concentrations of cytochrome c by one day of age. The less mature newborn rat did not attain comparable values until later in its development. The increase from the low fetal tissue concentrations to adult values occurred in the following sequence: heart concentrations increased earliest, followed by brain, skeletal muscle, and liver. Renal concentrations were the last to rise. Changes in cytochrome c concentration could be related to functional changes within these tissues as well as the over-all pattern of maturation represented by each of these species.