Plasma concentrations of desferrioxamine and ferrioxamine were measured following bolus injections of desferrioxamine and during 24 h infusions of the drug. [59Fe]ferrioxamine clearance and urinary iron excretion were also measured. Higher plasma ferrioxamine concentrations were found in iron loaded subjects and higher desferrioxamine concentrations in subjects with normal ironloads. There is a correlation between the circulating concentration of ferrioxamine during an infusion and the 48 h urinary iron excretion. The data suggests that the amount of iron chelated in vivo is related to an increase in the size of an intermediate chelatable pool rather than the total amount of the iron load. The well-recognized delay in urinary iron excretion appears to be related to active tubular reabsorption of ferrioxamine.