Chemical exchange dynamics expected for diffusive transfer of a chemical between aqueous solution and plant shoots, and expected bioconcentration based on partitioning properties of the chemical, are explored by using a three-compartment model. The model utilizes three dynamic compartments--leaves, stems, and aqueous exposure medium. Chemical mass flux is dependent on the morphometry of the plant species, on exposure concentration, and on partitioning character of the chemical. The steady-state bioconcentration factor, K(sub SH), is dependent on composition of the plant species and partitioning character of the chemical. The model was parameterized for three species of aquatic plants (Myriophyllum spicatum, Bacopa caroliniana, and Hydrilla verticillata) and two chlorinated benzenes (1,2,3,4-tetrachlorobenzene (TCB) and 1,2-dichlorobenzene (DCB)). Predictions of uptake and bioconcentration are compared to results of static laboratory studies conducted with whole shoots of the three species over exposure periods ranging from 1 h to 21 d. Shoots approach steady-state tissue burdens between 1 and 3 d after initial exposure. K(sub SH) values for TCB (360-750), the more hydrophobic chemical, are at least one order of magnitude greater than those measured for DCB (11-28).