The four types of non-parenchymal sinusoid lining cells can be identified in situ and can be isolated for in vitro studies: the fenestrated endothelial cells, the phagocytic Kupffer cells, the fibroblast-like fat-storing cells, and the 'natural killer' pit cells. In situ the last three sinusoidal cell types can be seen to proliferate and increase in number in experimental models of liver pathology, such as inflammations and fibrosis. The understanding of the population dynamics of these cells--their mitotic activity, turnover time, and migration--may therefore have clinical applications. In the present article our data concerning the proliferation kinetics of Kupffer and pit cells in rat liver are reviewed.