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Summary An adenocarcinoma in the seminal vesicles of a 15-month-old male Wistar rat from a 30-month inhalation study is described. The rat was killed because of cachexia, apathy and a large palpable mass in the abdominal cavity. Macroscopic examination of the abdominal cavity revealed a 3.8 cm × 3.2 cm yellow-grey to pink mass, firm to soft in consistency. The cut section revealed cystic spaces. Histologically, the mass consisted of epithelial cells arranged in glandular and solid patterns with abundant amounts of connective tissue. Epithelial tumour cells were round-to-cylindrical with round-to-oval basophilic nuclei and one or two prominent nucleoli and a distinct eosinophilic cytoplasm. The glandular structure contained clusters of macrophages in their lumen with eosinophilic cytoplasm and indented nuclei. Extensive necrosis and reactive inflammation were present. The histological features of the small nodules in the pancreas and on the surface of the liver, rectum and urinary bladder resembled those of the primary tumour in the seminal vesicles. Based on these criteria, the neoplasm (mass) was diagnosed as an adenocarcinoma of the seminal vesicles. The immunohistological examination confirmed the diagnosis, i.e. immunostaining was positive for cytokeratins (4, 7, 14, 15, 18, and 19), vimentin, PCNA, and ED1.