The midgut which changes dramatically from a functional digestive tissue in the feeding larva to a non-functional digestive tissue of the non-feeding larva and pupa, so-called a closed system has been regarded as a vestigial tissue or degenerating tissue (although it posseses excretory function and other unknown functions). Although little is known about biochemical changes, especially protein and nucleic acids, in the midgut during metamorphosis, histological studies on the midgut during postembryonic development have been reported. The present study describes changes in composition and quality of protein and nucleic acid in the midgut during metamorphosis in order to examine the relationship between structural and functional changes and biochemical changes in the midgut during metamorphosis. The amount of protein in the midgut changed in parallel with the wet weight of the midgut. The amount of RNA changed in similar to that of protein content, but the amount of DNA was on a constant level during metamorphosis. The soluble protein in the midgut at the fourth instar larval stage, the fifth instar larval stage and the beginning of the pupal stage were examined by electrophoresis in acrylamide gel. The 23 bands were detected in bufferend saline extracts of the midgut in the both larval and pupal stages. There were a number of quantitative and qualitative changes in the protein pattern between the fourth instar larva and the fifth instar larva. The bands 6 and 14 at the fourth instar larval, disappeared at the early stage of the fifth instar larva and the new bands 3 and 9 appeared during the fifth instar larval development. At the prepupal stage, the 3, 12 and 19 bands appeared newly and then the 7 to 10 bands were not distinguished, because each of these bands diffused as one band, although the many proteins occurred almost in the midgut in the larval and pupal stages. The protein pattern in the pupal stage was simple compared with that of the larval stage. It was found that the degree of the increase and decrease of the wet weight and protein and nucleic acid contents in the midgut follows closely the degree of feeding activity of the larva. It is suggested that those changes reflect changes in the structure and function of the midgut at the metamorphosis from larva to pupa.