The rate of RNA synthesis in the midgut of Bombyx mori during the period of larvalpupal development was assayed in vitro with isolated nuclei. The synthesis was examined with or without ecdysterone added in the assay medium, and the products were analyzed by means of polyacrylamide gel electrophoresis. The rate of RNA synthesis in nuclei isolated at the gluttonous stage (day 3 of the 5th instar) was 2.5 times as high as that in nuclei from the mature larvae (day 7 of the 5th instar), and the

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... species synthesized in the two preparations of nuclei were different. Both syntheses were stimulated by ecdysterone added in the reaction mixture, but features of the hormonal effect were different between the two larval stages. In case of the gluttonous stage, new species of RNA xvere found to be synthesized in ecdysteronetreated nuclei. On the other hand, nuclei from the mature larvae showed only quantitative hut not qualitative stimulation by the hormone. Many of the ecdysterone-stimulated RNA species found in midgut cell nuclei from the gluttonous larvae were similar to those found in nuclei from the mature larvae.