The present study reports the induction, histopathology, immunocytochemistry, growth pattern and proliferative behaviour of mesenchymal tumours of the urinary bladder of rats induced by a single intravesical dose (5 mg/kg/body weight) of N-methyl-N-nitrosourea (MNU). In 14 of 283 female Wistar rats (incidence: 4.9%). 16 non-epithelial tumours had developed after an experimental period of 14 months. The most common histological type induced was of fibrohistiocytic origin (eight benign-appearing and three malignant fibrous histiocytomas). Furthermore, two pure histiocytomas (one benign and one malignant) and three capillary and cavernous haemangiomas were produced. Since no metastases occurred and no clear-cut distinction between a merely expansive and a truly invasive growth was possible, the diagnosis of malignancy was based on the degree of cellular atypia and the mitotic activity. The benign-appearing fibrous histiocytomas showed a mean mitotic index of 0.06% and the malignant fibrous histiocytomas of 0.34%. The mitotic activity of the malignant histiocytoma was threefold (0.78%) as high as in the benign-appearing histiocytoma (0.25%). There exist close morphological similarities between the induced mesenchymal tumours in the rat and those occurring in the human bladder. Although the spectrum of histological types of mesenchymal tumours seen in the rat bladder was narrower than that encountered in man, MNU seems suitable for further studying the histogenesis, histopathology and biology of experimentally induced non-epithelial bladder neoplasms to gain information for a better understanding of the pathogenesis of human disease.