The role of Nigella sativa L. (Ranunculaceae) (NS) and Urtica dioica L. was investigated (UD) in the prevention of carbon tetrachloride (CCl4) induced liver fibrosis and cirrhosis. Fifty Sprague-Dawley rats were allocated into five groups (I, IIA and B, IIIA and B) and CCl4 was injected biweekly to all groups. Group I (control, CCl4 only), group IIA and B (NS fixed oil and volatile oil), group IIIA and B (UD fixed oil and UD decoction extract) rats were killed at the end of week 12 and histopathological and immunohistochemical examinations of liver tissues were performed. In the control group, coagulation necrosis and hydropic degeneration were marked in the periacinar regions (zone 3) associated with fibrosis in the periacinar regions and in the portal tracts. In groups IIA-B and IIIA-B (NS and UD), none of the serious histopathological findings were detected except for sparse coagulation necrosis in the periacinar regions. ASMA-positive perisinusoidal cells with myofibroblastic transformation and lysosomal enzyme activity suggesting fibrogenesis were also significantly more common in the control group than in the NS and UD groups. UD and NS seem to be significantly effective in the prevention of carbon tetrachloride induced hepatotoxicity in rats. Copyright (C) 2003 John Wiley Sons, Ltd.