These experiments were conducted to determine the effect of cataractogenic doses of microwave radiation at 2.45 gigahertz (GHz) on the lens epithelium of the rabbit. One hour before animals were killed, tritiated thymidine was injected into the anterior chamber of both eyes at postirradiation intervals varying from six hours to one month. Epithelial peels were made and autoradiographic techniques used to identify cells manufacturing DNA. Comparison of counts from both experimental and control epithelia revealed two patterns, depending on the presence or absence of vesicle strings. Those lenses without vesicle strings showed an initial pronounced suppression of mitotic activity followed by gradual return to normal levels. Those lenses with strings showed a precipitous rise in DNA synthesis on the fourth to fifth day after irradiation. This increased activity may be the result of lens hydration.