Marked cytochemical changes were observed in the cells of the lymph nodes and spleen of one hundred and twenty-seven albino rats observed over periods of two hours to thirty-two days following a single dose of 600 r total body x-radiations. The intensity of the cytochemical reactions for pentosenucleoprotein, protein-bound sulfhydryl groups, and alkaline phosphatase decreased within two hours after x-radiation. This decrease in the cytochemical reactions occurred very rapidly and preceded any discernible morphologic alteration. Lymphocytes that were extremely sensitive to x-ray exhibited in most instances irreversible cytochemical changes. The cells that survived the lethal effects of x-radiations exhibited an increase in intensity of the reactions for pentosenucleoprotein, protein-bound sulfhydryl groups, and alkaline phosphatase between twelve hours and four days following irradiation. The reactions for desoxypentosenucleoprotein and mucopolysaccharides were unaltered in the cells of the lymph nodes and spleen following x-radiation.