The changes of portal pressure were studied in adult mongrel dogs, while sple-nic blood flow was let increase in five steps from 0 to 350 ml/min. When intrahepatic portal circulation was left intact, there was no appreciable change in the portal pressure concomitant with changes of splenic blood flow, but in dogs with obstruc-tion of intrahepatic portal branches by silica particles, increase in splenie blood flow caused a marked elevation of the portal pressure, The role of the increase in splenic blood flow and the obstruction of intrahepatic portal branches in the pathogenesis of portal hypertension in idiopathic splenomegaly was discussed.