Although aquatic mosses are widely used as metal biomonitors in rivers, there are few effective models to describe metal uptake and loss by these plants. To fill this gap, we exposed the aquatic moss Fontinalis dalecarlica for 28 d to three Cd concentrations (approximately 5-50 nM) in a flow-through laboratory system. Cadmium accumulation by F. dalecarlica was rapid during the first few days of exposure and slowed thereafter but did not reach a steady state within our 1-month long experiment. This lack of a plateau in moss concentrations suggests that, for biomonitoring purposes, the duration of moss exposure should be considered either through a model of the type that we tested or by standardizing the exposure time of mosses transplanted in the field. During the subsequent 22-d elimination phase of our experiment, Cd concentrations in mosses did not return to their initial levels. This result suggests that a two-compartment model is likely to be more effective at describing Cd losses than would a one-compartment alternative. Indeed, predictions of a two-compartment model closely fitted our experimental data, which augurs well for the wider use of this model for other moss species and metals.