Trace metal bioavailability is often evaluated on the basis of steady-state models such as the free ion activity model (FIAM) and the biotic ligand model (BLM). Some of the assumptions underlying these models were verified by examining Pb and Zn uptake by the green microalga Chlorella kesslerii. Transporter bound metal ({M-R-cell}) and free ion concentrations ([M2+]) were related to experimentally determined uptake fluxes (J(int)). Although the BLM and FIAM correctly predicted Pb uptake in the absence of competing ions, they failed to predict competitive interactions with Ca2+, likely because of modifications of the algal surface charge and the active nature of Ca2+ transport. Zinc transport is also active; in this case, both the internalization rate constant (k(int)) and the equilibrium constant for the binding of Zn to the transport sites (KZn-Rcell)varied as a function of [Zn2+] in the bulk solution. For this reason, Zn uptake could not be modeled by the steady-state models either in the presence or absence of competitors (Cd and Ca). Furthermore, the role of Cu on Pb and Zn adsorption and uptake could not be predicted by either model because of secondary effects on the algal physiology and membrane permeability. Finally, a 17degreesC reduction in temperature resulted in a two to fivefold decrease in membrane permeability of the metals, an observation that also is unaccounted for by either the FIAM or BLM. This paper emphasizes the limitations of the models in well-controlled laboratory systems with the goal of extrapolating the results to complex environmental systems.