One hundred twenty-two of 225 accurately located and measured mammary segments were transplanted successfully into the isologous female mice, and intact or castrated male host mice treated with estrogen and progesterone pellets. Five of 42 segments that had ligatures placed to determine the orientation of the segment were also recovered. Fewer grafts were recovered from intact male hosts than from castrated male or intact female hosts. The lengths of transplants ranged from less than 0.2 mm to 3.0 mm. Transplants that were 0.6 mm long grew most frequently. Transplants obtained from different portions of any one mammary gland or from any mammary glands were recovered at comparable rates. The fourth mammary gland-free fat pads, the pararenal fat pads, the mesometrial fat layers and subcutaneous areas were used as transplantation sites. The ultimate size of the regenerated transplants was determined by the amount of adipose tissue in the transplantation site and the hormonal environment, but not by the original size of the transplant. Mammary transplants obtained from all portions of ducts, even from quiescent glands of a 734-day-old mouse, or grafts which had been ligated with silk or hair, showed equal capability of regeneration. All successful transplants responded to either endogenous or exogenous hormones in a manner comparable to the hosts' own mammary glands, and lactated at parturition.