The metabolism of estrone sulfate has been studied in normal, young males by single injections of 6,7-3H-estrone sulfate (9 subjects); by constant infusions of 6,7-3H-estrone sulfate (8 subjects), 6,7-3H-estrone (5 subjects), and 6,7-3H-estradiol (5 subjects); and by the measurement of the concentration of radioactivity in plasma as estrone sulfate, estrone, and estradiol. The disappearance of radioactivity as estrone sulfate following the single injections of 6,7-3H-estrone sulfate can be described as a function which is the sum of two exponentials. The initial rapid component (T½ = 3 min) represents spread into and transfer from a space, with a volume of 7.2 rh 0.6 (se) L. The mean value for the rate constant of total removal (reversible and irreversible) from this space is 266 ± 2 7 (se) units/day of which 11.1 ± 2.4 (se) % is irreversible. The mean metabolic clearance rate (MCR R) is 105 ± 20 (se) L/day/m2 Using the constant infusion technique the mean MCRr is 80 ± 10 (se) L/day/m2. The overall mean MCR of estrone sulfate is 90 ± 10 (se) L/day/m2. The mean transfer constant [ρ]S,1BB or fraction of 3H-estrone sulfate infused which is measured as estrone in the blood is 0.15 with 95% confidence limits of 0.12 to 0.18. In the two subjects in whom 3H-estradiol was measured the [ρ]S,2BB was 0.022 and 0.044. For the infusion of 3H-estrone and 3H-estradiol the mean [ρ]1,SBB and [ρ]2,SBB (95% confidence limits) were 0.40 (0.21 to 0.81) and 0.42 (0.30 to 0.58). Using the available data for PBE1S and [ρ]bb values, it is probable that most of the estrone sulfate entering the blood each day does so as a result of conversion from the free estrogens and little, if any, arises as a result of direct secretion.