The changes in liver haemodynamics following partial hepatectomy (PH) were studied using an 85Kr clearance technique to measure perfusion in the livers of rats before and after both PH and sham operations. Post-operative measurements were made for 8 h following operation and also at 21 and 35 h, when uptake of tritiated thymidine into DNA is maximal in regenerating rat liver. Sham operation did not influence hepatic perfusion, arterial pressure or portal pressure. PH induced an early increase in perfusion of 293% in spite of a decrease in arterial blood pressure during the initial 4 h. The increase in perfusion lasted at least 8 h, had returned to normal values by 35 h, and was not explained solely by an observed elevation of portal pressure of 50% which returned to normal 5 h postoperatively. The total volume flow to the liver remnant after PH, however, was essentially the same as the flow to the liver before operation.