The effects of cimetidine on the hepatic and portal vein blood flow were investigated in 20 cirrhotic patients with portal hypertension by computed sonography. Cimetidine given intravenously at a dose of 0.4g significantly increased the hepatic vein blood flow (60.2% +/- 63.6%, P < 0.01), at the same time its calculated cross section area increased by 40.2% +/- 81.3% (P < 0.01). The portal vein blood flow increased by 53.2% +/- 52.3% (P < 0.01). While its cross section area was 16.8% +/- 17.3% (P < 0.01). These results showed that the histamine H2-receptor antagonist cimetidine may dilate liver vascular bed and reduce portal blood flow resistance in cirrhotic patients with portal hypertension.