In previous studies we have shown that hepatocytes stream from the portal tract toward the terminal hepatic vein. The present study provides evidence that littoral cells participate in the same cell stream and that the liver actually streams en masse. Littoral cells stand for sinusoidal endothelia and Kupffer cells. Thirty male adult rats, random bred, were injected with 0.5 microCi [3H]-thymidine, specific activity 5 Ci/mmol/g body weight. The rats were killed in groups of five, at the following times: 1 h, 14, 30, 60, 90 and 120 days. The livers were processed histologically and dipped into liquid emulsion for autoradiography. In each animal 50 labelled hepatocytes and 50 littoral cells were randomly selected and their distance from the nearest terminal portal tract rim was measured. Both cell populations renew their cells continuously. Each consists of two cell types, progenitors, residing around the portal tract up to the distance of 200 microns, and functional cells, which inhabit the rest of the acinus. The two regions where the different cell types reside are known respectively as progenitor (P) and functional (Q) compartments. Both cells are formed in the P-compartment and advance jointly along a trajectory, the tissue radius, toward the terminal hepatic vein where they die. They progress at a daily velocity of 2 micron. Since both advance at the same speed, as long as they exist they remain neighbours. Liver parenchyma and stroma thus stream en masse.(ABSTRACT TRUNCATED AT 250 WORDS)