The plant Mentha piperita, or peppermint, is commonly used in the treatment of loss of appetite, common cold, bronchitis, sinusitis, fever, nausea and vomiting, and indigestion as a herbal agent. In this study, we aimed to investigate biochemical and histological effects of M. piperita Labiatae, growing in the Yenisar Bademli town of Isparta city, and Mentha spicata Labiatae, growing in the Anamas high plateau of the Yenisar Bademli town, on the rat liver tissue. Forty-eight male Wistar albino rats weighing 200-250 g were used for this study. Rats were divided into four groups of 12 animals: Group I received no herbal tea (control group); Group 11 received 20 g/L M. piperita tea; Group III received 20 g/L M. spicata tea; and Group IV received 40 g/L M. spicata tea. Herbal teas were prepared daily and provided at all times to the rats during 30 days as drinking water. Liver function tests, including aspartate aminotransferase (AST/GOT) and alanine aminotransferase (ALT/GPT) activities were measured. To evaluate liver antioxidant defences, superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase (CAT) and thiobarbituric acid reactive substance (TBARS) activities were determined in the homogenates of liver tissue. In addition, liver tissues were submitted for histopathologic examination. AST and ALT activities were increased in Group 11, Group III and Group IV gradually when compared with the control group. The difference between Group 11 and the control group was not statistically significant (P > 0.016). Increases in AST and ALT activities of Group III and Group IV were statistically significant when compared with the control group. SOD, GSH-Px and CAT activities were increased in Group 11 when compared with the control group but the difference was not statistically significant (P > 0.016). However, SOD, GSH-Px activities and the TBARS level were significantly increased, and CAT activity was significantly decreased in Group III when compared with the control group. In Group IV, while SOD, GSH-Px and CAT activities were decreased, the TBARS level was increased as compared with the control group (P < 0.0016). Histopathological evaluation of experimental groups revealed a mild to severe degree of hepatic damage when compared to the control group. In Group II, there was only minimal hepatocytes degeneration. In Groups III and IV, there were granular or ballooning hepatocyte degeneration and necrosis, sinusoidal and central vein dilatation. It was concluded that lipid peroxidation and hepatic damage occurs after M. piperita and M. spicata administration in rat liver and the damage seems to be dose dependent.