Endogenous peroxidatic and phagocytic activities were studied on the sinusoidal lining cells in the perfusion fixed rat liver under normal and reticuloendothelial system (RES) stimulated conditions. All cells with cytostructural characteristics of the Kupffer cell showed endogenous peroxidase activity in the endoplasmic reticulum (ER) and perinuclear cisternae, while the endothelial cells were always negative in peroxidase reaction. When colloidal carbon or zymosan were given intravenously, Kupffer cells exhibited stickiness of their cell surfaces against these particles and avid phagocytic activity with the extension of pseudopods, but endothelial cells did not show any stickiness and incorporated carbon particles only a very small amount. These findings were obtained not only under the normal condition but also RES stimulated condition, and, on the basis of peroxidase reaction and phagocytic activity, no intermediate type of cells between the Kupffer cell and endothelial cell was found. Four days after RES stimulation by the admini-stration of estradiol or typhoid vaccine, the relative number of Kupffer cells to parenchymal cells increased about two-fold, but that of endothelial cells did not change significantly. The results presented in this article did not support the hypothesis that the endothelial cell might be a precursor of the Kupffer cell, but suggested that each of them might be the distinct line of cell.