In normal rat liver, Kupffer cells were unequivocally identified using peroxidase cytochemistry by light microscopy in semithin plastic sections. The Kupffer cell population was found to constitute 31% of the sinusoidal cells and by morphometry and serial sectioning, a mean absolute number of 14 to 20 X 10(6) Kupffer cells per g liver was calculated. The mean distribution of Kupffer cells in the liver lobules was 43% in the periportal, 28% in the midzonal and 29% in the central area of the lobule. Administration of latex particles labeled only 64% of all Kupffer cells, and in particular centrally located cells, showed a lower activity of latex uptake, even at overloading doses. Furthermore, the latter cells were of smaller size than periportal Kupffer cell profiles. The mean number and distribution of latex-labeled Kupffer cells did not change over a period of 3 months, indicating a long lifetime for these resident macrophages. This slow population turnover was supported by the observed small mitotic index, 0.06% after a 6 hr arrest by vinblastine, and by the small [3H]thymidine labeling index which did not change over a period of 3 weeks after administration of the label. It is proposed that the Kupffer cell population, under physiologic conditions, is a long-living and self-renewing population, the kinetics of which substantially differ from those of other sinusoidal cell types.