1. As in man, the plasma iron concentration in rats exhibits a circadian variation, with an early morning peak and an evening trough. 2. The release into the plasma of iron liberated from haemoglobin within the reticuloendothelial cells of the liver and spleen was studied by injecting heat-damaged erythrocytes containing 59 Fe-labelled haemoglobin. 3. The percentage of 59 Fe released was lower in the morning than in the evening. 4. These observations suggest that a circadian variation in reticuloendothelial cell behaviour influences the plasma iron concentration.