In onion root-tip cell nucleoli some tapered cylindrical structures appear in the dense fibrillar component (DFC) after cycloheximide treatment. These structures resemble the "Christmas-tree" like configurations which have been noted in spread preparations from the nucleoli. The similarities in their general appearance are strengthened by observations after RNase and pepsin digestions. The presence of such structures in the DFC of prophase nucleoli has also been demonstrated. These observations indicate the presence of active ribosomal cistrons in DFC of the nucleolus. It is proposed that nucleolar transcription becomes initiated in the DFC around the FC (fibrillar centres) and then these transcripts along with their axis move to the granular component (GC) for their processing. As such some of these structures were observed to penetrate into the GC. © 1993 Academic Press. All rights reserved.