Uptake and accumulation kinetics of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and its two major N-nitroso metabolites, hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX), in earthworms was investigated. Results indicated that RDX and its N-nitroso metabolites were rapidly absorbed into earthworms (Eisenia fetida), reaching the highest concentrations within a few days. Accumulation of RDX was greater than its N-nitroso metabolites, as evidenced by a higher bioconcentration factor (BCF); BCFs were 1.86, 0.39, and 0.05 for RDX, MNX, and TNX, respectively. RDX and its N-nitroso metabolites were also rapidly eliminated from the earthworm and/or transformed to other metabolites, as evidenced by the rapid decrease of test compounds in earthworms after reaching their highest concentrations. The uptake of MNX and TNX increased as exposure concentration increased. Although these earthworms might (anaerobically) degrade RDX to MNX and MNX to TNX, it is hypothesized that this process would be slow. Other biotransformation pathways may be involved in biodegradation of RDX and its N-nitroso metabolites due to the fact that concentrations of tested compounds decreased in both soil and earthworms. It is hoped that these data can be used to refine environmental management strategies for RDX and for performing specific risk assessments of RDX and its N-nitroso metabolites.