1. Injection of 0-16,ug. of actinomycin D into pupae of the beetle Tenebrio molitor L. results in the development of modified adults in which the head and thorax are essentially adult while the abdomen and wings remain pupal-like. It is suggested that the messenger RNA for the development of head and thorax is present in the animal from the first day of pupation. 2. Injection of 0 16,ug. of actinomycin D brings about 51-67% inhibition of labelled uridine incorporation into RNA. 3. When thymus DNA is mixed with actinomycin D before injection into pupae the latter develop into normal adults. This protection does not occur when DNA and actinomycin D are injected separately. 4. The inhibition of incorporation of labelled uridine into RNA by actinomycin is diminished to some extent when DNA and actinomycin D are injected separately and abolished if they are injected together. 5. Inhibition of RNA synthesis by actinomycin D in vitro is fully reversible. DNA or deoxyguanosine can reverse the effect of actinomycin D. 6. Incorporation of labelled glycine into protein is not affected by actinomycin D injection during the first 6 days of pupation. On the seventh day it becomes diminished in control pupae but this effect is prevented by actinomycin D. It is suggested that the template for protein synthesis is stable during the first 6 days of metamorphosis and that on the seventh day there is a qualitative change in the protein synthesized on the template.It was noted in 1958 that actinomycin D inhibits RNA, but not DNA, synthesis in Bacillus subtilhs (Slotnick, 1958), and interferes with RNA synthesis in HeLa cells (Milton, Goldstein, Slotnick, Hillman & Gallagher, 1959). Reich, Franklin, Shatkin & Tatum (1961) showed that actinomycin D inhibits the incorporation of tritiated cytidine into RNA, but does not interfere with the incorporation of tritiated thymidine into DNA of L cells in tissue culture. They further showed in strains L-929 and L-2 mouse fibroblasts that actinomycin D will suppress the biosynthesis of normal cellular RNA and not the biosynthesis of DNA and protein for prolonged periods. A binding takes place between actinomycin D and template DNA