The ultrastructure of the ducts and subareolar ducts of the resting female breast have been described. In transverse section ducts have longitudinal folds, some of which are solid ridges of cells. Four distinctive cell types were distinguished; epithelial cells, lymphocyte type cells, basal clear cells and myoepithelial cells. The epithelium is single layered, multiple at folds, and similar in general morphology to the terminal ductal-lobular unit. Well developed terminal bars may prevent cellular disruption during contraction, and apical cytoplasmic vesicles represent possible secretory material. Basal cytoplasmic bodies may represent transport of secretory products into or from the periductal stroma. Intranuclear vesicles may also be linked to secretory activity. The myoepithelium is well differentiated with numerous cytoplasmic filaments and 9+0 cilia, forming a discontinuous layer around the epithelium. The basal lamina is generally multilaminate. Capillaries are 1-5mum in diameter and exterior to the delimiting fibroblasts of the epithelial-stromal junction.