Bioassays were carried out to study the differences in cadmium (Cd) toxicity to premetamorphic tadpoles of Rhinella arenarum, in aqueous solutions with variable contents of calcium in the presence and absence of humic acids, and to analyze the relationship between the free Cd2+ ion concentrations calculated by chemical modeling and the biological results. The correlation analysis of the free Cd2+ concentration calculated by chemical speciation and the analytical Cd yielded a direct relationship between the degree of toxicity and the slope value. The lowest slope was obtained from the treatments with lowest free Cd2+ ion concentration and lowest toxicity, whereas the highest slope was obtained from the most toxic treatment. At comparable concentrations of free Cd2+, intralarval Cd increased as the Ca in the solution decreased. At equal contents of Ca, in the presence of humic acids, the content of Cd in larvae was higher and the toxicity values lower. The results obtained in this study show that waterborne Ca could offer some protection from metal uptake and accumulation by competitive inhibition in the uptake mechanism that involves active transport via cell membrane. In the systems with humic materials, a certain proportion of the Cd present in the solution was associated to them and thus became less bioavailable.