1. The conductance of the rat lens was measured using a two‐internal‐microelectrode technique. The voltage response to a step of current consisted of two components arising from bulk and membrane resistance respectively. 2. The potassium permeability was calculated by applying Goldman theory to 86Rb+ efflux data. 3. The internal pH (pHi) and internal free calcium (pCai) were measured directly using single‐ and double‐barrelled ion‐sensitive microelectrodes. 4. Lens pHi was 6.9 in control solution (external pH, pHo = 7.3) and was reduced on lowering pHo. The presence of propionate or 100% CO2 in the external solution accentuated this effect. 5. Internal acidification was accompanied by a depolarization of membrane potential, an increase in membrane and cell‐to‐cell resistance and a decrease in potassium permeability. The acidification had no effect on pCai. 6. The intracellular pH was increased by perifusing with trimethylamine or NH4Cl. Both treatments induced a membrane depolarization with little change in potassium permeability. Subsequent removal of NH4Cl led to a sustained decrease in pHi. 7. In every case where pHi decreased, the changes in membrane potential and conductance could be explained largely on the basis of a decrease in potassium permeability. The concomitant increase in cell‐to‐cell resistance was less pronounced and probably insufficient to uncouple the lens system.