Two groups of mice were irradiated to the upper body with the same total dose but with different duration of radiation exposure. Following irradiation leukocyte count hair graying weight loss and mortality were observed. Leukopenia was more profound and prolonged in the group irradiated over the longer duratioa Conversely mortality, weight loss and hair graying showed the expected dose rate effect with high intensity short-duration radiation being more effective. Two independent dose-rate effects seem possible with partial-body irradiation Those cells that stay within the irradiation field (hair follicles and oral mucosa) showed an expected dose-rate effect; the reverse is noted for the peripheral leukocyte count where longer duration is more effective. This is consistent with the hypothesis that the longer radiation allows more circulating hematopoietic stem cells to enter the irradiated area and be destroyed.