Before the fish were transferred to the exposure aquaria, duplicate water samples 1 ml were taken from each aquarium, after mixing of the water by aeration, to check that Cd was evenly distributed in the water. Two additional samples were taken at the end of the 24-h exposure period to check for possible changes in Cd concentrations during the exposure. The cadmium concentration in the water decreased only 6-10%. The slight decrease in the Cd concentration was mainly due to adsorption of Cd to the exposure aquarium and net basket.