Summary Galactose-fed rats with unilateral cataracts develop cataracts in the second eye even though they are placed on a normal diet. Rats fed a 35% galactose diet for 14 days, followed by a normal diet, develop cataracts in a median time of 18 days just as if they were continuously on a galactose diet. The cataractogenic process becomes irreversible after 10-14 days of galactose. Restoration of a normal diet during the irreversible period results in a return of lens hydration, dulcitol and ATP to normal levels. It is suggested that changes such as these may be placed in the early or late period of cataractogenesis depending on whether they tend to return to normal or to continue their abnormal trend. Thus, the vacuoles seen after 2-4 days of galactose which are reported to be due to fiber swelling and bursting develop parallel with but are not the cause of the later white opaque cataracts.