The quantitative and qualitative changes of transcriptionally active nucleolar organizer regions (NORs) identified by silver staining (AgNORs) were observed in rat tongue lesions induced by 9-10 dimethyl 1-2 benzanthracene (DMBA). Epithelial hyperplasia, mild, moderate dysplasia and severe dysplasia (with or without invasive squamous cell carcinoma) were induced 4,6,8 & 10 weeks respectively after implantation of 5 mg DMBA in the submucosa of rat's tongues. A general trend towards an increase in the mean AgNOR counts with longer time and resultant histopathological findings was observed, where the lowest value (2.44 +/- 1.00) was found in specimens showing epithelial hyperplasia (4 weeks after DMBA implantation) and the highest value in those showing severe dysplasia with invasive squamous cell carcinoma (10 weeks after DMBA implantation). Clumping and irregularity of AgNOR dots were demonstrated in DMBA treated specimens and were found to increase with higher degrees of epithelia dysplasia. The dispersal of the AgNOR dots in the nucleoplasm indicating high cellular activity was characteristically evident in specimens showing severe dysplasia with invasive squamous cell carcinoma. Our results suggest that although AgNORs can be useful as a qualitative marker of cellular activity at the early stages of DMBA tongue carcinogenesis, counting the AgNOR dots cannot be relied upon in discriminating the different degrees of induced cellular changes.