When LAF1 and C57BL mice are exposed for the duration of life to daily doses of Co60-gamma-rays ranging from 6 r/day to 210 r/day, the extremities progressively darken. With doses ranging from 6 r/day to 170 r/day the degree of hyperpigmentation appears to be a function of both radiation intensity and total accumulated radiation dose. Shielding experiments show that hyperpigmentation results from a direct action of X- and gamma-radiation. Radiation-induced hyperpigmentation involves an increased amount of epidermal melanin associated with increased melanogenic activity of melanocytes in the basal layer of the epidermis. Graying of the pelage, indicative of a loss of follicular melanocytes, also occurs in daily irradiated mice with hyperpigmentation of the extremities. The bearing of these observations on general problems of pigment cell dynamics is discussed.