The cell population kinetic parameters defining a simple model of the recognizable part of the erythroid system have been determined. Experimental results using tritiated thymidine and radioactive iron autoradiography have provided estimates of the number of cell divisions, transit times and flow rates for all the recognizable stages of the erythroid system. The accuracy of the estimates and the validity of the model employed are discussed.

Summary: Estimates have been obtained of the parameters defining the cell population kinetics of the erythroid system in rats subjected to protracted anaemia and to continuous γ‐irradiation at 43 rad/day. The inflow of cells to the proerythroblast compartment and the outflow of erythrocytes to the circulation have been measured by 55Fe autoradiography. In the anaemic rats, the ratio of the outflow to the inflow is normal, indicating that the precursors undergo the normal number of divisions during maturation from proerythroblast to erythrocyte. In the continuously irradiated rats, the outflow is normal but the inflow is decreased by a factor in the range 2–4, so that the ratio of the outflow to the inflow is increased by this factor, and the cells undergo more divisions than normal during maturation. The cell proliferation parameters of the various maturation stages of the erythroid system have been determined in anaemic rats, using 3H‐TdR autoradiography. The number of divisions undergone during maturation through each stage is normal, but the maturation times and cycle times are considerably reduced. Less information is available on the proliferation parameters in the continuously irradiated rats, but the 55Fe experiments indicate that the transit times of the proerythroblasts and of the non‐dividing stages of the system are increased in response to this stress.

Summary: The method of labelled mitoses has been used to obtain estimates of the cell‐cycle time (and of its constituent phases) of erythroid precursors in normal adult rats, and in adult rats subjected to transient and protracted anaemias. It has been shown that considerable reductions occur in the cycle time in the anaemic rats, and that these reductions are due to decrease in the duration of both the S and G1 phases. The decreased cycle time is discussed in relation to the number of divisions that the erythroid precursors undergo during maturation.

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