We gave three-weeks-old rats a diet which included galactose in three different concentrations (15%, 25%, 50%) producing sugar cataracts of three different degrees.Combining the 3H-thymidine autoradiographic method with the whole-mount preparations of total lens epithelial cells of one lens, we examined the proliferative activity of the lens epithelium in the early stages of cataract crisis (for 14 days). In the lenses of three-weeks-old normal rats, 3H-thymidine labelled cells were observed mainly in the proliferative zone at the lens equator, and at the same time a few were detected in other parts. On the first day after we started feeding the rats galactose-included diets, the very early stages of cataract formation and an increase in the number of labelled cells were observed in the proliferative zone when compared with the control group on a normal diet. On the fourth day, we found an increase in the number of labelled cells in the proliferative zone and a remarkable ectopic proliferation in other parts. However, in contrast to this, as the cataract progressed from the seventh day to the fourteenth day, we noticed a decrease in the number of labelled cells in the proliferative zone and in the other epithelial layers when compared with the control group. The peak in the number of labelled cells on the fourth day was observed regardless of the galactose concentrations in the diet, and was not proportional to the degree of the cataracts.In rat galactose cataracts, the cure phenomenon (Diet-Reversal), in which the lens opacity is removed by altering the galactose-including diet to the normal diet, was observed. This Diet-Reversal was proved through light microscopy (17). Moreover, we also reported that aldose reductase inhibitors (ART) had curative effects and preventive effects on galactose cataracts (1-3, 11, 12). On the basis of the above-mentioned results, we think it is essential that the cataract lens is repaired by the regenerated fibers at the lens equator. That is to say, the examination of the proliferative ability and the kinetics of the regenerated lens epithelium seems to be very important in order to develop the cataract therapeutic agents for the future, though it is impossible at present.We used the 3H-thymidine autoradiographic method on thin sections embedded with methacrylate, and proved that, in the developmental process of rat galactose cataracts, the lens epithelium will repeat the ectopic proliferation even in other parts than the lens equator, and play an important role in the repair of the regenerated lens fibers (13,14). In particular, on the fourth day after we started feeding rats the 50% galactose-including diet, a remarkable increase in the amount of ectopic proliferation was observed. But to give a conjecture on the whole layer of lens epithelium from only one thin section seemed to be unreasonable. Therefore, in this investigation we used the whole-mount preparations of the lens epithelial layer. In this study, we examined the ability of the rat lens epithelium to proliferate dur...