An ultrastructural study of hepatocyte proliferation in the regenerating rat liver has been made by means of the partial hepatectomy. And electron microscopic histochemistry of hepatocyte in the regenerating rat liver is studied through alkaline phosphatase reaction. The results are as follows: 1. When the regeneration of rat liver is induced by the partial hepatectomy, the prominent ultrastructural characteristics of hepatocyte are changes of the distribution of chromatin in nucleus, increase of the number of mitochondria and decrease of the size of them, development of rough endoplasmic reticulum, and transient decrease of glycogen granules in cytoplasm. 2. Alkaline phosphatase reaction products are appeared in the nucleus or rough endoplasmic reticulum of hepatocyte during the initial regeneration of liver as 24, 48 and 72 hour groups after partial hepatectomy. And these positive reaction are mainly increased in cytoplasm and plasma membrane of hepatocytes during 1, 2 and 3 week groups after partial hepatectomy. As 4 weeks passed after partial hepatectomy, these positive reaction is located in the sinusoidal epithelial cells or erythrocytes. With above results, we concluded that alkaline phosphatase was synthesized in the rough endoplasmic reticulum bounded ribosomes of regenerating hepatocyte, was transported to the plasma membrane of them, and then was transported in blood by the way sinusoidel epithelial cells.