A new analytical procedure for the assay of DDS was based upon extraction into organic solvents and measurement of fluorescence in anhydrous ethyl acetate. Men receiving single 100-mg oral doses of DDS showed peak plasma levels of 1.1 to 1.5 µg per ml in 2 to 4 hours; the levels then dropped to half-values in 20.6 hours, and to a few millimicrograms per ml by the 5th day. Intramuscular doses of DADDS administered in an oil vehicle produced peak plasma levels of about 0.06 µg per ml in 6 days, and “plateau” levels of 0.035 to 0.030 µg per ml within 2 weeks. This level was maintained for about 60 days after dose, and dropped slowly thereafter with an apparent biological half-life of 42.6 days. The decrease in plasma levels occurred some 50-fold more slowly with DADDS than with orally administered DDS. The urinary excretion of “total” DDS ran parallel to the blood levels in both studies; it accounted for 68% of the oral dose of DDS and about 54% of the intramuscular dose of DADDS.