One day following radioiron injection, the concentration of the isotope in erythrocytes of control hamsters was 14.1 times the concentration in hibernators' erythrocytes during the second day of hibernation. During this same period, hibernators' plasma contained 4.62 times the control concentration of radioiron. At definite times after tagging, the activity of chromium51-labeled erythrocytes was measured in warm- and cold- exposed animals and in hamsters after measured periods of hibernation. In the active animals, the potential erythrocyte half-life was approximately 40 days; the effective half-life was about 15 days. Retardation of senescence and the virtual absence of random destruction in erythrocytes of hibernating hamsters increased the potential erythrocyte life to approximately 160 days. Both the minimal metabolic activity of the animal and the retardation of intrinsic senescent processes in erythrocytes were considered to be responsible for the diminished erythropoietic stimulus in hibernating hamsters. Radioiron transport across the cell membrane, its incorporation into hemoglobin, and mitotic activity of narrow erythroid elements were component processes of erythropoiesis which were probably retarded.