Two experimental approaches have been used to determine whether the internal fibres of the amphibian lens have intact membranes. One approach involves an analysis of the potential transients obtained when potassium is substituted for sodium in the external medium and the other involves a correlation of lens electrical resistance with depth of penetration of the measuring electrodes. The half-times of the potential transients were in the region 100–120 sec and taking 4 × 10−6 cm2 sec−1 as the diffusion coefficient of potassium, these values correspond to a diffusion path length of 200–350 μ. As an “unstirred layer” of the surrounding medium probably extends for 100–150 μ from the lens surface, the potential-determining membranes lie at most 100–200 μ below the surface. As the diffusion of potassium through the capsule and through the extracellular space of the lens is probably much slower than in free solution, this distance may well be reduced to 10–20 μ. The resistance, measured by the two-electrode method, was found to be independent of the depth of the electrodes below the surface. There was also no marked difference in the resistances of the anterior and posterior faces of the lens. These resistance results also suggest that the ion-limiting membranes lie near the surface.