The metastasizing pleomorphic adenoma of the salivary glands is a rare variant of pleomorphic adenoma with a benign microscopical appearance, but malignant biological behaviour and production of metastasis. The histopathological study with the routine techniques is not enough for disclosing the nature of these tumors and they are most of the times underdiagnosed as benign. The correct diagnosis, almost always too late, is evident after several recurrences and the detection of metastatic foci. The number of nucleolar organizing regions (NORs) detected with the silver colloid method can be used in histopathology to determine the degree of cell activity. We report the findings in a case of metastasizing pleomorphic adenoma of the submandibular gland. This technique demonstrated to be useful for the diagnosis and characterization of the metabolism of these tumors.