1. 1. Cd, Zn, Hg and methyl mercury chloride (on anterior, not posterior) decrease the potential difference and short-circuit current across the lens of the toad. 2. 2. When translenticular Na transport was abolished, these metals were still effective. When Na was removed from the anterior, the effects of Cd and Zn were abolished, whereas Hg and CH3 Hg continued to act. Lenses in vitro accumulated Na and lost K in the presence of Cd and Hg but Zn or CH3Hg had no effect. 3. 3. O2 consumption in vitro was reduced by Zn, Hg and CH3Hg, but not Cd. In the absence of Na, Hg and CH3Hg still decreased O2 consumption indicating a general toxic effect. 4. 4. The group IIB metal ions alter the electrical parameters and permeability of the crystalline lens but they act in different ways which reflects their varied chemical properties.