The study evaluates quantitative cytological and histological changes in spleen white and red pulp of rats continuously irradiated for up to 140 days with daily dose rates ranging from 0.5R--82.5R. Significant reduction of lymphocyte incidence begins at the rate of 5R/day and the decrease deepens with the rising dose rate until a steady state around 5 per cent is reached at 82.5R/day. Small lymphocytes are relatively more affected. Irradiated animals show a wave-like rising of ectopic erythropoiesis in spleen red pulp from day 30 on even with the lowest dose rates of 0.5R and 1R/day. The ratio or relative incidences of haemocytoblasts to erythroblasts increases approximately twofold, indicating that a substantial reduction of stem cells is compensated to a large extent already at an early stage of erythropoiesis. Beginning with the 40th day of exposure, the incidence of myeloid elements also rises, the increase being more apparent at medium-range dose rates for eosinophils than neutrophils. Both the red and white spleen pulps of exposed animals display an increased incidence of plasma cells.