An attempt was made to recognize by subgross morphology with dissecting microscope magnifications any abnormalities in the canine mammary glands and then to classify these structures by histopathologic criteria. Techniques developed for demonstrating hyperplastic lesions in mammary glands of rodents were applied to whole mammary glands of untreated female beagle dogs. 8 intact purebred female beagle dogs were obtained from the Radiation Biology Laboratory at the University of California in Davis where they had been maintained since birth. Any unusual structural change as compared to the normal lobularity of the mammary gland was considered an atypical nodule. Such nodules were classified histologically as normal inflammatory or proliferative. There were 763 atypical nodules studied histologically. Evaluation of the mammary gland of the dog by the techniques described could shorten considerably the time required and improve the accuracy of oncologic and drug evaluation studies. Recognition of preneoplastic changes in the canine mammary gland could have useful applications in the study of tumor biology and might have significant relationships to the current study of the human mammary gland. This might include development of a more reliable and consistent diagnostic technique for early mammary tumors.