The histology and fine structure of 7 human mammary glands were studied in the postpartum. Widespread hypertrophy and hyperplasia of the mammary acini were observed accompanied by dilatation and engorgement of the lumen by milk. Loose strands of connective tissue surrounding the mammary lobules and ductules contained dilated, engorged vascular channels. The lactogenic epithelial cells displayed rich cytoplasm containing prominent layering stacks of rough endoplasmic reticulum closely related to the enlarged oval mitochondria, on occasion surrounding their entire circumference. Also noted were numerous ribosomes and polyribosomes. The markedly hypertrophied Golgi apparatus revealed cisternae containing particulate electron dense material and vesicles with dense granules. The latter were frequently seen being discharged in the lumen devoid of limiting membrane. Abundant fat droplets were present in the basal and apical regions and “pinched off” into the lumen, apparently surrounded by a limiting membrane. The lactogenic cells were richly endowed with slender microvilli in their luminal pole and related to one another by cytoplasmic prolongations, desmosomes and tight junctions. The myoepithelium was stretched and thinned out toward the periphery of the acini between the lactogenic epithelium and the basement lamina. Abundant myofilaments were present in the myoepithelial cells. Pinocytotic vesicles were also present, although not very numerous, in both the lactogenic and the myoepithelial cells. In general the human mammary gland during lactopoiesis follows similar structural changes as those previously described in other mammals. (J Clin Endocrinol Metab40: 834, 1975)