The fine structure of the oviduct epithelium of the newborn to the old mouse was studied with an electron microscope. Just after birth, epithelial cells lining the ampulla of the mouse oviduct are simple columnar in shape and of one type in fine structure. They contain numerous free ribosomes, an extremely poor rough endoplasmic reticulum, and a small Golgi complex. In the 3-day-old mouse, the epithelial cells are differentiated neither into ciliated nor secretory cells, and are characterized by the appearance of many autolysosomes and a solitary cilium. The ciliary cells differentiates 5 days after birth. Ciliogenesis is frequently observed at 5-10 days. The important role of the fibrous granules for ciliogenesis and that of the Golgi apparatus for membranogenesis of the cilia are described and discussed. The secretory cell having mucous secretory materials is differentiated at 23 days. In the epithelial cell lining the ampulla of the aged (22 to 24-month-old) mouse oviduct, large autolysosomes and vacuoles 2-6 micrometer in diameter occur in the ciliated cell, though cilia and other cell organelles are well preserved. In the old mouse the secretory granules almost disappear and the rough endoplasmic reticulum is strikingly dilated in the secretory cell. No features showing the transformation between the secretory cell and the ciliary one are seen in the mouse oviduct.