To investigate the role of heme oxygenase(HO), a catalyzing enzyme of heme to produce CO, in modulation of systemic circulation in CCl4-induced cirrhotic rats. Saline(vehicle) and ZnPP were s.c. injected into the posterior necks of rats respectively and the rats were then anesthetized by pentobarbital sodium in four hours. Mean arterial pressure (MAP, kPa), heart rate (HR, b/min) and portal pressure (PP, cm/H2O) were measured by indwelling catheter. Plasma CO was determined by Chalmers method. Heme oxygenase acivity was determined by the rate of bilirubin formation. The cirrhotic rats showed significant hyperdynamic circulation indicated by decreased mean arterial pressure [MAP, (15.6+/-1.7) vs (18.9+/-0.9) kPa, t = 4.52, P less than 0.01] and increased portal pressure [PP, (16.7+/-0.8) vs (8.8+/-0.3) cm H2O, t = 23.10, P less than 0.01] as compared to normal control rats(NS). ZnPP could cause a significant increase in MAP [(17.3+/-1.5) vs (15.6+/-1.7) kPa, t = 2.18, P less than 0.05] and significant decrease in PP [(13.2+/-0.7) vs (16.7+/-0.8) cm H2O, t = 8.53, P less than 0.01] in cirrhotic rats. The cirrhotic group presented a significant increase in plasma CO [(18.0+/-1.9) vs (10.4+/-1.3)mumol/L, t = 8.42, P less than 0.01] and HO activity in the spleens [(11.1+/-0.9) vs (6.5+/-0.9) nmol bilirubin/mg protein/h, t = 9.28, P less than 0.01] and intestines [(2.5+/-0.1) vs. (1.3+/-0.2) nmol bilirubin/mg protein/h, t = 15.1, P less than 0.01]. ZnPP could cause significant decreases in plasma CO and HO activity in liver, spleen and intestine of both control and cirrhotic rats. HO-CO system activation may be an important reason for the hemodynamic disturbance of liver cirrhosis.