The post- and preembedding ultrastructural localization of transcribing rRNA genes has been carried out in nucleoli of permeabilized onion growing root tip protoplasts by means of the nonisotopic bromouridine method. By means of both post- and preembedding approaches, major synthetic sites were identified with morphologically distinct subdomains of dense fibrillar components, with some signal also being associated with nucleolar fibrillar centers and vacuoles, Moreover, labeled medusoid fibrils within distinct domains seen in Lowicryl thin sections likely represent the morphological correlate of transcribing nucleolar genes. (C) 1996 Academic Press, Inc.