A study was undertaken to investigate the pharmacokinetics of an organically bound form of selenium. Six adults received a single oral 200-micrograms dose of 74Se as L-selenomethionine. A kinetic model was developed to simultaneously account for the appearance and disappearance of the tracer in plasma, urine, and feces. The model included absorption distributed along the gastrointestinal tract, uptake by the liver-pancreas subsystem, enterohepatic recirculation, distribution to two large tissue pools, and transport through four components of the plasma pool. Average turnover time of the plasma components varied from 0.01 to 1.1 d. The turnover time in the liver-pancreas subsystem ranged from 1.6 to 3.1 d. Turnover time ranged from 61 to 86 d in the peripheral tissues with the slowest turnover. The whole-body residence time was approximately five-fold greater than the turnover time of the tissue pool with the slowest turnover, reflecting substantial reutilization of labeled material.