The thiols of structural water-soluble proteins of the rabbit lens are normally in reduced form. In vitro the surviving lens of normal eyes is able to maintain the proteins in the reduced form even in the presence of high oxygen tension, so long as the capsule and the epithelium are apparently uninjured and glucose is available in the incubation medium. When the capsule is removed and glucose is not added, there is a fall of free glutathione and oxidation of water-soluble proteins. Experimental uveitis causes a very early change in conformation of structural lens proteins, which makes them more reactive leading to formation of artifactual disulphide bonds during the preparation procedure. The surviving lens of ‘uveitic’ eyes seems not to be able to maintain the reduced form of proteins in vitro in the presence of high oxygen tension. In this case, free G-SH, which is unchanged, does not appear to protect protein thiols against oxidation.