Many studies on the substances that control the insect metamorphosis have done recently and Butenandt has successed to obtein the "verpuppungshormon" in the crystall form from silkworm pupa in 1954. However, the progress of the body constituents in insect tissue during metamorphosis is not yet enoughly explained. In this paper the investigations about the relation between tissue prctein and free aminoacid during metamorphosis of Cu'ex pipiens were carried out to explain certain parts of above question. The result is shown on the Fig. I. The results can be summarized as follows : Free amino-acids in tissues are much aboundant in 3 and 4-instar larvae and they decrease rapidly just after pupation. Although the free amino-acids content increases slightly in pupal stage, it is relatively constant during this stage and similarly it does not show any notable changes after emergence. On the other hand the tissue protein is most scant in 3-instar larvae and it increases rapidly in accordance with their developement to the 4-instar larvae and pupation. During pupal stage the tissue protein content is constant generally and it is also similar after emergence. Such rapid decrease of free amino acids and the rapid increase of protein in tissues during pupation suggest the vigorous histogenesis for producing the new tissues in this stage. And during the pupal stage contents of free amino-acids and protein are both constant in general. This constancy seems to show the histolysis and histogenesis progress in this stage relatively constant or may utilize chiefly the free amino acids during pupation and glycogen and fat substances during pupal stage. Quality of free amino acids in each stages of 4-instar larvae, pupae (initial stadium), pupae (end stadium), adults (initial stadium) are shown on Table I, estimated by paper chromatography. It shows the presence of : in 4 instar larvae 18 free amino-acids (α-alanine, glutamic acid, leucine & isoleucine, proline, histidine, serine, valine, methionine, lysine, arginine, aspartic acid, glycine, tyrosine, phenylalanine, cystine, hydroxyglutamic acid, methionine sulfoxide), in pupae of initial stadium 11 (α-alanine, glutamic acid, leucine & isoleucine, histidine, valine, elysin, arginine, aspartic acid, glycine, tyrosine), in pupae of end stadium 8 (α-alanine, glutamic acid, leucine & isoleucine, valine, arginine, glycine, tyrosine), and in adults 10 (α-alanine, glutamic acid, leucine & isoleucine, proline, histidine, valine, lysine, arginine, glycine) respectively. These results seem to indicate also that the free amino acids content in 4-instar larvae is much predominant as compared with them in the pupal or adult stages.