Chloride channels are known to be involved in the regulated volume decrease that occurs when the rat lens is exposed to hypotonic challenge. We now report that chloride channel blockage makes the rat lens gain water under isotonic conditions, suggesting that chloride and water fluxes may also play an important role under resting conditions. Histological comparison of hypotonically and isotonically swollen rat lenses revealed a significant difference: in the former, fibre cells were swollen from the periphery inwards, while in the latter, swollen fibre cells were confined to a discrete cortical zone which was located 150-200 micron from the lens surface with cells on either side of this zone appearing unaffected. This localised fibre cell swelling is remarkable because of its similarity to the situation in the diabetic rat lens.