Adenomyosis is a nonneoplastic hyperplastic lesion, characterized by invagination of proliferating endometrial glands into myometrium. In dogs, uterine adenomyosis is relatively rare and it is important in Toxicologic Pathology to differentiate other non-neoplastic and neoplastic lesions in uterus. In the present study, we report two cases of adenomyosis in the female beagle dogs used for a chemical toxicity test. Clinically, one out of the two female beagle dogs, 15 months of age, had vaginal bleeding for 2 weeks and the other one, 11 months of age, showed swelling of vulva for a week. At necropsy, the weight of uterus was markedly increased to 27.9 g and 15.8 g, compared with the mean value (4.01±2.37, n=6) of that of other normal dogs, respectively. The parameters of hematology and serum chemistry were ranged normal in both of the dogs with enlarged uterus. For differentiation of connective tissue with muscle fibers, Van Gieson stain was also performed in the serial tissue sections. Histopathologically, the lesions of the enlarged uteruses were characterized by proliferating endometrial glands into myometrium, surrounded by connective tissue. The endometrial glands were proliferating downward to myometrium or embedded in multiple clustered glands in deeper myometrium without compressing the adjacent muscle fibers. The gland epithelial cells are uniformly cuboidal shape with a dense and bottom-located nucleus. These gross and histological findings were consistent with adenomyosis.