The changes in the activities of constitutive nitric oxide (NO) synthase (cNOS) and inducible NOS (iNOS) were investigated in arterial tissues of CCl4-induced cirrhotic adult SD rats. The aortic tissue homogenate were prepared in normal and cirrhotic rats. NOS activity was measured by conversion of 3H-arginine to 3 H-citrulline. The activities of cNOS and iNOS were calculated in terms of presence or absence of Ca2+. The results showed that activities of total NOS, cNOS and iNOS in arterial tissues were all increased significantly in cirrhotic rats as compared with those in normal controls. There was a significant positive correlation between the activities of total NOS and cGMP content in cirrhotic arterial tissues.