The fate of C-14-2,4,6-trinitrotoluene ([U-C-14]TNT) in soil/plant systems was studied using onion (Allium cepa L.) plants with only a single root. It was found that the single roots grew exponentially and that the rate of water uptake of the onion plants increased exponentially, as well. The concentration of [U-C-14] in the roots at first increased and then appeared to reach a steady state, while the [U-C-14] concentration in the leaves was found to increase linearly with time. The [U-C-14] concentration in the rhizosphere increased gradually, while in the bulk soil it decreased slowly. The accumulation of [U-C-14] in the rhizosphere is likely to difference between movement into the rhizosphere (through advective mass flow of soil water by root uptake) and its uptake into the roots. The distribution of C-14 in the soil/plant system was found to be 60-85% in the soil solid phase, 7-11% in the soil liquid phase, <1% in the soil air phase, <1% in the root compartment, and <0.01% in the leaf compartment. The maximum RCF (root concentration factor) value for TNT and its derivates was found to be about 20, and the maximum TSCF (transpiration stream concentration factor) was 0.18. These values can be changed by a variety of factors in soil-plant systems.