Cd2+ (10-3m) on the external side of the frog skin, increased the electrical potential difference (PD), short-circuit current (SCC), and resistance across this membrane. The effect was irreversible. Cd2+ (10-3 m) on the serosal side of the toad urinary bladder inhibited the SCC, but this was increased by 10-3 m and 10-4 m at the mucosa or 10-4 m at the serosa. The increases in SCC were inhibited by amiloride, indicating the effect was due to a stimulation of active transmural sodium transport. The increased resistance seen in the frog skin may reflect a decreased permeability to chloride. These results indicate that cadmium can influence active ion transport and the permeability of epithelial membranes.