摘要：RNA metabolism in the posterior silkgland of the silkworm, Bombyx mori L, during the fifth instar has been studied. The base composition of the bulk RNA did not change significantly during the instar. 32 P-pulse labeling during the earlier stage of the instar indicated the synthesis of a rapidly turning-over RNA having a base composition different from that of ribosomal RNA and resembling that of DNA. Sedimentation analysis of RNA at this stage revealed the presence of components having different sedimentation constants from those of the ribosomal and s-RNA. At a later stage of the instar, there was no evidence for the occurrence of a rapidly turning-over RNA having a base composition different from that of ribosomal RNA. The role of RNA in silk fibroin synthesis is discussed.