During the conduct of a study designed to determine the effect of 3-methylcholanthrene (3-MC), a synthetic polycyclic aromatic hydrocarbon (PAH) that acts through the aryl hydrocarbon receptor (AhR), on uterine contractility in Wistar rats, uterine tumors were identified in both vehicle and 3-MC-treated animals. The objective of the current study was to describe the histological characteristics of these tumors. Sexually mature female rats (110 days old) were treated with 70 micro mol/kg 3-MC or vehicle (olive oil) for 4 days and euthanized by exsanguination. At necropsy uterine tumors were unexpected findings in two vehicle and four 3-MC-treated rats. The tumors appeared as multiple unilateral or bilateral subserosal nodes. No tumors were found in other tissues on gross inspection. Prior to necropsy, tumor-presenting animals were acyclic and arrested in a state of persistent proestrus. Haematoxylin and eosin staining of tumor sections revealed nests of acidophilic granule-containing cells within a highly vascular stroma of the uterine wall below the muscularis. Positive periodic acid Schiff (PAS) staining suggested the presence of glycogen or glycophospholipids within these granules, however, negative PAS diastase staining indicated that the acidophilic bodies were not composed of glycogen. The tumors are histologically similar to human dysgerminomas. We conclude that these tumors are unrelated to treatment and are of a granular type not previously documented in Wistar rats.