Abstract: Fragile X syndrome is the most common form of inherited mental retardation. It is typically caused by a mutation of the Fragile X mental-retardation 1 (Fmr1) gene. To better understand the role of the Fmr1 gene and its gene product, the fragile X mental-retardation protein in central nervous system functions, an fmr1 knockout mouse that is deficient in the fragile X mental-retardation protein was bred. In the present study, fragile X mental retardation 1-knockout and wild-type mice are used to determine behaviour and oxidative stress alterations, including reduced glutathione, oxidized glutathione and thiobarbituric acid-reactive substances, before and after chronic treatment with melatonin or tianeptine. Reduced glutathione levels were reduced in the brain of fmr1-knockout mice and chronic melatonin treatment normalized the glutathione levels compared with the control group. Lipid peroxidation was elevated in brain and testes of fmr1-knockout mice and chronic melatonin treatment prevents lipid peroxidation in both tissues. Interestingly, chronic treatment with melatonin alleviated the altered parameters in the fmr1-knockout mice, including abnormal context-dependent exploratory and anxiety behaviours and learning abnormalities. Chronic treatment with tianeptine (a serotonin reuptake enhancer) did not normalize the behaviour in fmr1-knockout mice. The prevention of oxidative stress in the fragile X mouse model, by an antioxidant compound such as melatonin, emerges as a new and promising approach for further investigation on treatment trials for the disease.