Dietborne metal uptake prevails for many species in nature. However, the links between dietary metal exposure and toxicity are not well understood. Sources of uncertainty include the lack of suitable tracers to quantify exposure for metals such as copper, the difficulty to assess dietary processes such as food ingestion rate, and the complexity to link metal bioaccumulation and effects. We characterized dietborne copper, nickel, and cadmium influxes in a freshwater gastropod exposed to diatoms labeled with enriched stable metal isotopes. Metal influxes in Lymnaea stagnalis correlated linearly with dietborne metal concentrations over a range encompassing most environmental exposures. Dietary Cd and Ni uptake rate constants (k(uf)) were, respectively, 3.3 and 2.3 times higher than thatfor Cu. Detoxification rate constants (k(detox)) were similar among metals and appeared 100 times higher than efflux rate constants (K(e)). Extremely high Cu concentrations reduced feeding rates, causing the relationship between exposure and influx to deviate from linearity, i.e., Cu uptake rates leveled off between 1500 and 1800 nmol g(-1) day(-1). L. stagnalis rapidly takes up Cu, Cd, and Ni from food but detoxifies the accumulated metals, instead of reducing uptake or intensifying excretion. Above a threshold uptake rate, however, the detoxification capabilities of L. stagnalis are overwhelmed.