Changes in the level of total proteins and protease activity in brain, muscle and liver tissues of normal and x-irradiated frogs were determir, ed. Low doses of radiation produced an increment in protein level while high doses produced decrement. However, protease activity at all doses exhibited an etevatory trend. Exposure of frogs to lethal doses resulted in increased protease activity and decreased protein content during post-irradiation periods. The results are discussed on the basis of protein destruction and lysosomal damage.