A circadian rhythm was demonstrated in 10 males and 10 females with respective mean decreases in plasma iron concentration at 18 hr of 62% and 47% of morning values. Ferrokinetic studies performed on 5 normal males and 5 normal females showed a more rapid disappearance rate and lower plasma iron turnover in the evening. Parallel studies were done on 6 normal males in the morning and 4 normal males in the evening of the release of reticuloendothelial iron at 8 and 18 hr after intravenous injection of 59Fe chondroitin ferrous sulfate. The 6-hr release in the morning was 54.1% and in the evening 25.9%. Composite data from morning and evening showed a correlation between plasma iron level and plasma iron turnover (r = 0.76, p less than 0.001). A similar correlation existed between the plasma iron level and the percent of radioiron released from the reticuloendothelial system (r = 0.67, 0.02 less than p less than 0.05). These data are consistent with a fluctuating iron release from the reticuloendothelial cell in normal subjects, which would account for the diurnal variation in plasma iron.