The livers of rats subjected to end-to-side portacaval anastomoses were studied 3 to 5 months postoperatively by in vivo and electron microscopy. Compared with sham-operated controls, the livers of portacaval anastomoses animals contained dilated, tortuous networks of sinusoids. The velocity of blood flow in these vessels tended to be slower and more variable than controls, but always progressed toward the hepatic venules. Blood entered the sinusoids from portal venules and from arteriosinus twigs which terminated in the initial segments of some of the sinusoids at the periphery of the lobule. Together, the arteriosinus twigs and the short, initial segments of these sinusoids formed functional arterioportal anastomoses. These, in combination with the lack of portal venous flow, resulted in retrograde blood flow in portal venules. Nevertheless, blood still flowed from these portal venules into the sinusoids unless the sinusoid was fed by an arteriosinus twig. In addition to these microcirculatory alterations, the number of Kupffer cells that phagocytized latex particles was less in the animals with portacaval anastomoses, as was the number of particles ingested by these cells. Scanning and transmission electron microscopy confirmed the paucity of Kupffer cells. Those seen appeared inactive since they were flattened, exhibited few microplicae and filopodia and contained few latex particles. The endothelial cells of the sinusoid lining were perforated by increased numbers of large fenestrate which may be a reflection of elevated intrasinusoid pressures generated by the expanded arterialization of the sinusoid bed. The observed dilated sinusoid network interspersed by narrowed plates of hepatocytes is also consistent with this hypothesis. Finally, scattered nodular foci were observed which contained enlarged hepatocytes, narrow sinusoids, active Kupffer cells, and more normal rates of blood flow. Such sites may represent attempts by the liver to regenerate its normal architecture.