With rats from which about 70% of the liver was previously removed, corrosion casts and specimens of freeze-cracking method were prepared from time to time after the operation. These specimens were used for scanning electron microscope observation. Ultra-thin sections were also prepared for transmission electron microscope observation to study regeneration of the sinusoid during regeneration of the liver. The results were compared with normal rat liver, and were as follows: Regenerative proliferation of the sinusoid after partial hepatectomy began about 12 hours after hepatectomy. First the space of Disse enlarged. This later becomes a sinusoid, so the author had designated it as a presinusoid space. The presinusoid spaces appear about 12 hours after hepatectomy, and can be observed up to about the third post- operative day. In the early stages, they are spherical, about 2-10 μ in diameter, and grow larger with lapse of time. Later they become larger than the sinusoid. Some have many bumps running parallel to the sinusoids, while others bind with the sinusoid to form bridges. They are located at the margin of the sinusoid facing one fixed direction. The sinusoid and the presinusoid space are separated by endothelial cells lying in between them. The other external wall is composed of liver cells. The number of microvilli of the liver cell surface is extremely small. This stage has been called the stage of the presinusoid space. In the presinusoid space, the liver cell surface which forms the exterior wall shows newly formed endothelial cells after cell division. These protrude and completely cover the inner surface. At the same time a neoplastic sinusoid forms by removing the endothelial cells there. The formation of the new sinusoid starts about 24 hours after partial hepatectomy and continues up to about the third postoperative day. This stage has been designated the sinusoid budding stage. The neoplastic sinusoid is at first closed at one end but later these neoplastic sinusoids fuse to form a new sinusoidal network. Such fusion commences about 48 hours after partial hepatectomy, and continues vigorously for about 3 postoperative days. This stage is called the fusion stage. The new sinusoidal network formed by the fusion of neoplastic sinusoids is irregular, and its surface is uneven, but with time, the arrangement and the surface contour become regular and smooth. The process is completed by about 14 postoperative days. This stage has been designated the completion stage.