Abstract The myoglobin content of the skeletal muscles of the rat was found to vary between 0.7 and 1.2 mg./g. fresh muscle. The content of cytochrome c which is extractable with water was found to vary between 0.0122 and 0.0187 mg./g. fresh muscle. The corresponding figures for the acid-extractable cytochrome fraction are 0.0141 and 0.0174. Prolonged irradiation with ultraviolet light did not influence the myoglobin and cytochrome c content. Incorporation of C 14 into myoglobin I and II does not significantly differ. The activity of 1 millimole glycine present in the globin of myoglobin of glycine-2-C 14 -injected rats is much larger than the corresponding activity of hemin. The pattern of activity decay with time suggests the presence of two populations of myoglobin molecules of different life spans. One has a half-life of 20 days, the other one of 80–90 days. The hemoglobin of our rats had a half-life of 50–60 days. A significant effect of irradiation with 300 rad. of Co 60 γ-rays on C 14 incorporation into the hemin of myoglobin was observed, not, however, on incorporation into the globin moiety.