The cell cycle time of erythroid cells was determined in normal mice and in polycythaemic mice after stimulation of erythropoiesis by exogenous erythropoietin. The method employed was 3HTdR autoradiography of bone marrow cells. After 21 or 38 hours of administratering the erythropoietin 3HTdR was injected into the animals. The cell cycle time was calculated from the curves of changes in the labelling indices of proerythroblasts and basophilic erythroblasts at one hour intervals. The cell cycle time in normal animals was 8.5 hours. Twenty-two to twenty-five hours after stimulation with 1.5 units of erythropoietin, the cell cycle was 7.8 hours. Thirty-nine to forty-two hours after injection, at the time when the number of nucleated erythroid cells was within normal limits, the cell cycle time was 7.7 hours. Under the used experimental conditions, the cell cycle time of early erythroid cells was not significantly changed in comparison with the values obtained in non-treated animals. This is in contrast to the results reported when extreme stimulation of erythropoiesis with endogenous erythropoietin was used.