Developmental changes in RNA content and its synthesis in the midgut during metamorphosis and the effect of actinomycin D on RNA synthesis in the midgut was studied. The large amount of rRNA and tRNA per gram of tissue through the period of feeding at the fifth instar were contained in the midgut at high constant value of approximately 4 mg and 0. 7 mg, respectively, and then decreased rapidly at the beginning of the larval maturation. The result implies that the midgut at glutoneous stage, unlike that at stage of larval maturation, has a high rate of protein biosynthesis. The rate of tRNA synthesis continued to increase gradually during the period of feeding and then enhanced greatly at larval maturation at which larva ceased to feed. On the other hand, the rate of rRNA synthesis continued to increase linearly accompanying larval development, that is, the rates of both RNA in the midgut at larval maturation were higher than that at the feeding stage. The higher concentration of actinomycin D was injected, the stronger RNA synthesis was inhibited. Inhibition of RNA synthesis appeared within 1 hour after administration with actinimycin D, reached maximum by 5 hour and then continued for 24 hours. Although the larva treated with high concentration of actinomycin D, such as 25 fig, was died within 5 to 6 hours after injection, even the administration of larva with 25 fig of actinomycin D did not inhibit completely RNA synthesis, but inhibited 80 per cent of the control. The differential sensitivity of rRNA and tRNA syntheses to actinomycin D was noted. Actinomycin D inhibited rRNA synthesis stronger than tRNA synthesis. Inhibition of both RNA syntheses was dependent on the actinomycin D dosage with which larvae were injected. After the 50 per cent blockage of further RNA syntheses by the treatment of the I-day-old fifth instar larva with actinomycin D, developmental changes of both RNA contents were examined through stage of the fifth instar larva. Ribosomal RNA and tRNA contents during the period of feeding were at constant value of about 3. 2 mg and 0. 6 mg, respectively, each value was about 75 per cent of the control, except for at the 5th day of the fifth instar, tRNA content was higher than the control. The results indicated that RNA synthesized during the fourth moulting and/or the fourth instar occurs relatively stable during the period of feeding at the fifth instar.