The resting pH of the rat lens was determined using H+-sensitive liquid membrane microelectrodes and found to be 6·89 when measured in a perifusing solution of pH 7·20. The pH of the rat vitreous humour was also measured and was found to be 7·25. Attempts were made to perturb the lens pH by varying the pH of the perifusate. In the presence of alkaline solutions, the lens was able to maintain its resting pH and membrane potential but. upon perifusion with a more acidic solution, the lenticular pH equilibrated with the pH of the bathing solution and the potential depolarized. The internal pH could be manipulated independently of the external pH by perifusing the lens with Ringer solution containing 20 mm ammonium chloride. The ammonium chloride induced a rapid alkalinization and the return to control solution caused a fall in pH to below the normal resting level. The biphasic pH response to ammonium chloride was accompanied by changes in lens transparency and membrane potential.