The nature of disulphide bonds in 3, 7 and 24 month old rat lens proteins has been determined in order to examine the possibility that there is a direct correlation between intermolecular disulphide bond formation and hardening of the lens during ageing.Significant levels of S-S were found in all of the crystallins, as well as in the insoluble proteins. However, no substantial changes in the molecular weight distribution of the proteins, or their constituent polypeptides, were observed after the reduction of these bonds. This indicates that almost all of the S-S bonds are intramolecular.It would appear that the hardening of the nucleus is not a result of intermolecular disulphide crosslinking during ageing. Instead, our observations suggest that intramolecular disulphide bonds may make the gamma-crystallin molecule more compact, thereby promoting closer packing in the nucleus where the concentration of this protein is highest.