Seven mammary glands from male and female human fetuses varying from 13 to 40 weeks gestation were studied ultrastructurally. At 13 weeks the mammary bud grows into the upper dermis, forming solid offshoots of epithelial cells surrounded by a well defined basement lamina and dense fibroblastic stroma; myoepithelial cells are not seen. The epithelial cells show small desmosomes and interlocking processes, large pools of glycogen and scant lipid droplets. At approximately 20 weeks, ductules are observed linked by epithelial cells in two possible states of differentiation. Myoepithelial cells are well defined, containing myofilaments, dense bodies and hemidesmosomes attached to a distinct basement lamina. During the last trimester the epithelial cells show both short and long slender microvilli on the free surface of the ductular lumen, sometimes accompanied by large cytoplasmic protrusions. The Golgi system and the abundant endoplasmic reticulum have dilated cisternae filled with finely granular material, some of which appear in the process of being discharged into the lumen. Abundant mitochondria and lipid droplets are observed. These ultrastructural observations substantiate the possibility that during fetal mammary growth and differentiation in utero, the variable endocrinological conditions occurring throughout gestation elicit initially a proliferation and conditioning of epithelial cells which are rendered functionally active and secretory sometime toward the second trimester of pregnancy. These morphological findings are interpreted in the light of our current knowledge of the fetal and maternal hormonal conditions and interactions, particularly in regard to mammotrophic hormones.