Several studies have shown that a large number of amphibian species are capable of tolerating a wide range of external osmolaritles (Bentley 1971; Duellman and Trueb 1986). However, the mechanisms which respond to variations in the osmotic environment are different in larvae and adults. They are also dependent on the way that the osmotic pressure is reached, i.e., through substances which behave as ionic or no-ionic ones In solution (Ballnsky 1981). The eurihalinity of tadpoles was found in species of both Urodela and Anura. The tolerated concentration rate was wide, from distilled water to several hundreds of mOsm. In the particular case of Bufo arenarum, we have demonstrated that its young tadpoles are able to survive when immersed in distilled water, NaCI and mannitol solutions of considerably higher osmolarlties than its natural environment (Ferrari and Sallbián 1967; Ferrarl et al 1988), suggesting the existence of adaptive physiological mechanisms. We evaluated the impact of Cd(ll) on those mechanisms In the present study. Cadmium is a contaminant widely distributed in freshwater ecosystems (Ravera 1984) and known as an element that interferes in epithelial ionic permeability processes (Hayashi et al. 1977; Hillyard et al. 1979; Takada and Hayashi 1980). We hypothesized that as a consequence of those properties, Cd(II) may alter the already observed ability of Bufo arenarum tadpoles to overcome important changes in the physicochemical characteristics of their incubation media. We evaluated the effect of a sublethal concentration of this metal on the water balance of the animals immersing them in different solutions. Evidence is presented that inorganic ions have shown a protective action to the toxic effects of Cd(II).Facultad de Ciencias Naturales y Museo