Rat lenses incubated in hypotonic medium adapted to the medium by first undergoing osmotic swelling and then returning after 1 to 2 days to their originial volume. Two processes--osmotic swelling and volume regulation--appear to be working simultaneously. The former is indicated by a decrease in the concentration of cell potassium to the expected level and the latter by a limitation of lens volume associated with a decrease in the potassium content of the lens. These findings are consistent with those observed in other types of cells and may be of significance in understanding the etiology of cataracts.