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

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... inly 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.