The protective action of thymol (paramethyl-isopropyl-phenol) was investigated against carbon tetrachloride (CCl 4 )-induced hepatotoxicity in male Swiss albino mice. The CCl 4 at a dose of 20 μl kg −1 produced damage to liver cells and was followed by the significant increase (P<0.001) in serum alanine aminotransferase (ALT) activity and hepatic lipid peroxidation after 24 h. The hepatocellular necrosis was further confirmed by histopathological examination of liver section. Oral administration of thymol in a single dose (300 mg kg −1 ) resulted in significant (P<0.05) amelioration of CCl 4 -induced hepatotoxicity. Thymol also inhibited lipid peroxidation induced by CCl 4 in vivo. The protection offered by thymol was also evident from histopathology photomicrograph. In a separate in vitro assay, thymol inhibited the non-enzymatic lipid peroxidation of normal mice liver homogenate induced by Fe 3+ -ascorbate. The present study suggests that thymol protects the liver against CCl 4 -induced toxicity and the protection may be mediated through its ability to inhibit lipid peroxidation. However, other interactions between thymol and CCl 4 remains to be elucidated. 1999 Academic Press@p$hr