Melatonin is a well-known antioxidant substance. The attention paid to its adverse effects is limited. Moreover, data published about its toxicity and /or undesired effects are scarce in the literature. The main aim of the present study was to evaluate subacute melatonin effects on antioxidant enzymes, lipid peroxidation and liver transfera es in experimental animals. Female Wistar rats were used in this study and divided into two groups. Group I was a control group and received corn oil. Group II was treated with 100 mg/kg (oral) melatonin for 7 days. The antioxidant enzyme activities such as superoxide dismutase (SOD), catalase (CAT) and lipid peroxidation marker malondialdehyde (MDA) levels were measured. Alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), cholesterol and urea levels were also measured. Additionally, histopathological evaluations of hepatic tissue samples were performed. The obtained results showed that the subacute melatonin exposure reduced both antioxidant CAT activity and MDA levels (both, p<0.05). However, ALT and AST levels increased in melatonin treated rats (both, p<0.05). The ratio of AST to ALT in the melatonin treated group was nearly two-fold higher than the control group. Microscopic examination revealed no severe histopathological changes in the liver. In conclusion, melatonin treatment causes changes in female rats liver functions and further investigations are necessary to examine the full impact of melatonin supplementation with regards to route of administration, dose as well as the exposure period.