Changes of the action of ribonuclease and protease in the course of polyhedrosis development were pursued using acetone preparations prepared 4 and 6 hours as well as 2, 3 and 6 days after viral infection. It was found that the proteolytic activity is elevated in all cases, and that the maximum activation occurs in the period of polyhedron formation. The elevation of ribonuclease action, however, was rather low. The enzymatic potencies o f larvae were also estimated several hours after injecting with virogenic agents, for example, hydrogen peroxide, hydroxylaminc, potassium nitrite, acetoxime, sodium carbonate and polyhedral virus. It was observed that the tissue protease is activated by all these treatments. But the increase of ribonuclease activity took place only when caterpillars were injected with nitrite, oxime, carbonate and virus.