An anaplastic carcinoma was found in one of the two parathyroids of a 2-year-old male Wistar rat, which was sacrificed at the end of a carcinogenicity study. Morphologically, it was characterized by the presence of nodular areas of pleomorphic and dense cells with numerous atypical mitoses and large regions of smaller and dark monomorphic cells devoid of mitoses and forming small cystic spaces. Local invasion of the capsule and pronounced compression of the parenchyma of the thyroid gland were observed. Immunohistochemically, the tumor was markedly positive for the parathyroid hormone and negative for the thyroid transcription factor. The proliferative activity was assessed by immunostaining the endogenous cell proliferation associated-antigen Ki-67, and the proliferating cell nuclear antigen. The diagnosis of carcinoma of the parathyroid was made on the basis of microscopic and immunohistochemical findings.