A common model for producing experimental liver cirrhosis is the administration of CCl4 to phenobarbital (PhB)-stimulated rats. However, concern may arise due to the complex actions of PhB upon liver metabolism. This study examined the role of PhB in the production of CCl4-induced liver cirrhosis in the rat. In addition, regenerative capacity of the liver after partial hepatectomy (PH) or portal branch ligation (PBL) was studied in cirrhotic rats, rats treated with CCl4 alone, and in PhB-treated controls. In rats given PhB throughout the CCl4-induction period, ascitic form of micronodular cirrhosis was found in 93% with only 3% mortality. In contrast, rats pretreated with PhB for only 2 weeks followed by CCl4 alone for 18 weeks did not develop liver cirrhosis. In most of the cirrhotic rats, PH induced hepatic regeneration associated with improved liver histology. PBL was less effective. Treatment with PhB alone for 10 weeks resulted in liver atrophy and reduced hepatic regenerative capacity. Impaired regeneration response was also found in rats treated with CCl4 alone. In conclusion, treatment with PhB throughout the CCl4-induction period seems necessary for the production of liver cirrhosis in rats. However, prolonged treatment with PhB alone results in liver atrophy and an impaired regenerative response. Therefore, though necessary for the cirrhotic model, PhB by itself has negative hepatotrophic influences which questions the thoroughness of the PhB/CCl4 experimental model.