ABSTRACT. The rates of uptake of cadmium and zinc by the amphipod crustacean Orchestia gammarellus (Pallas) increase with decrease in salinity from 36.5 to 25"% NaC1, as expected from physicochemical changes in the ava~labilities of free metal ions. Between 15 and 2.5':~~ NaCl cadmium and zlnc uptake rates plateau, and the cadmium uptake rate falls at 12% NaC1. This pattern of change of uptake rate with salinity change is not dependent on trace metal exposure concentrations, and cannot be explained by uptake via any enzyme-driven uptake route. It is concluded that at low salinity the amphipods effect one or more physiological responses that offset any increases in cadmium and zinc uptake rates expected from physico-chemical increases in the availabilities of free metal ions at low salinity. Such physiological responses are induced by changes in total osmolality, as opposed to inorganic salinity, and are not maintained on transfer from low to high osmolalitv. The physiological response is not explicable only in terms of change of the uptake rate of calcium, nor only in terms of change in apparent water permeab~lity which may play a role at extremely low salinities. In low salinity the amphipods do excrete newly accumulated cadmium and zinc but this excretion does not explain the lack of increase in net uptake of cadmium and zinc at low salinities The identification of the physiological response of 0. gammarellus reducing trace metal uptake at low salinity remains enigmatic, and may turn out to be combinat~on of several effects.