Over the last several years, environmental contamination with cadmium has significantly increased because of its extensive use In anthropogenic activities. This heavy metal is a very toxic xenobiotic producing reproductive and developmental impairments in a wide spectrum of organisms. Within the life cycle of organisms, the embryo is the most sensitive period to adverse conditions. Moreover, stage-dependent susceptibilities to toxic agents in amphibian embryos treated with lead, cadmium and aluminium were described. In the case of cadmium, this differential sensitivity could be related to changes in the metal accumulation through development or in the induction of defense mechanisms against cadmium toxicity, such as metallothionein (Mt) synthesis, which seems to be developmentally regulated. In the case of the toad Bufo arenarum, susceptibility to cadmium seems to follow a biphasic pattern during embryonic development. From the two-cell stage to the neurula stage an increase in susceptibility occurs, whereas from the last stage onwards a gradual increase in the resistance against this heavy metal seems to be achieved. This stage reports the uptake profile of cadmium at different post-hatching stages. 20 refs., 3 figs.