Hyperosmotic concentrations of NaCl, mannitol, urea, and ethanol decreased the electrical potential difference (PD) and short-circuit current (SCC) across the amphibian lens in vitro. NaCl and mannitol were most effective and their effects were reversible. The osmotic agents were only effective when placed on the anterior side of the lens, which is the site of the lens epithelium. The PD across the anterior side of the lens was decreased by the osmotic agents, and the resistance across this side increased. A small decline in the PD was observed across the posterior side under these conditions. The decline in the translenticular PD and SCC was substantially reduced, but not abolished, when Na was excluded from the anterior bathing solution. Absence of Na from the posterior solution also decreased the response. When the anterior side of the lens was bathed with Na-free Ringer's, mannitol produced a much larger increase in resistance then when Na was present. The lens accumulated Na in the presence of mannitol and ethanol, but there was no concomitant decline in K. The extracellular space was unchanged, so that the Na was in the cells. The translenticular flux of 22Na in the direction anterior to posterior was increased, while that from posterior to anterior was not changed significantly. Lens opacities were observed to occur when the lens was bathed on the posterior surface with mannitol, but this was not associated with electrolyte changes. The osmotic agents appear to influence the electrical activity of the lens by decreasing the activity of the Na-pump in the anterior lens epithelium and increasing its passive permeability to Na.