The rhino mouse, so called because of its characteristic rhinocerous-like skin cannot raise its young. Although the animal has rudimentary mammary glands, these are thought to be defective due to their impossibility to raise their young. Morphological observations of the female mammary glands of rhino mice (hrrh/hrrh) were carried out and the results were compared with those of their normal litter mates (hrrh/+) at various ages from 1 month to 1 year. No morphological differences in mammary gland development was seen between the rhino mice and their normal litter mates, and normal secretory changes were observed in the rhino mice after delivery. Mouse mammary tumor virus (MMTV) expression and the occurrence of hyperplastic alveolar nodule (HAN) were analogous to these features in phenotypically normal hrrh/+ mice. Further, the sweat glands in the rhino mouse were normal. Thus, anomalies in the rhino mouse appeared to be limited to the skin and pilosebaceous apparatus.