Separation of mammary ducts from the nipple or the primary lactiferous duct, and resection of the mammary glands, except the portion proximal to the nipple, did not interfere with the growth of the remaining portion of the mammary gland. One hundred and fifty-one segments of the upper four pairs of mammary glands of virgin mice were transplanted into either male or female isologous hosts at four different sites—the pararenal fat pad, the dorsal subcutaneous area, the retroauricular fat-pad area, and the fourth mammary-gland-free fat pad. Female donors and both male and female hosts of the CBA and C57BL strains and their reciprocal CC hybrid groups were used. The transplanted mammary tissue grew equally well in all isologous combinations and parental to hybrid combinations. Transplantability, except for the size attained by the grafts, was not affected by the age or sex of donors and hosts, or the site of transplantation. Successfully transplanted mammary glands responded to either endogenous or exogenous hormones in a manner similar to the controls. They developed fully and lactated at all sites after parturition of the host, regardless of the extent of growth. The transplants at the four sites were similar histologically. The extent of growth was controlled by 1) the amount of adipose tissue at the transplantation site available for growth of the transplants, 2) the period of transplantation, and 3) the hormonal environment. The first factor was the essential one. The adipose tissue transplanted with the mammary parenchyma did not grow, but often diminished in size and was replaced by differing amounts of connective tissue of the host.