Wholemount techniques developed for demonstrating atypical nodules in the mammary glands of rodents and dogs were successfully applied to entire mammary glands of female nonhuman primates. Histologic findings in the nodules were focal or diffuse epithelial hyperplasias and hyperplastic lobules with normal proportions and appearance of cellular elements. Morphologic evaluation of the nonhuman primate mammary gland by the techniques described appears to be more critical than postmortem examinations commonly in use and would be of particular value when breast changes constitute a significant part of the necropsy findings. Similarities exist between certain of the lesions described and those suspected as preneoplastic in the human breast.