Abstract: N-Methyl-N-Nitrosourea (NMU) is a highly specific mammary gland carcinogen that act directly and does not require metabolic activation. The novel medicinal plant Morinda citrifolia, also called as Noni, has broad therapeutic effects such as antibacterial, antiviral, antifungal, anticancer, analgesic, anti-inflammatory, anti-oxidant and immune enhancing effects. The present study was conducted to assess the beneficial effects of M. citrifolia fruit juice on antioxidant, hematological and biochemical alterations caused by NMU induced mammary carcinogenesis in Sprague-Dawley rats. The rats were divided into five groups viz., vehicle control group-A (n = 8), M. citrifolia control group-B (n = 8), NMU control group C (n = 15), M. citrifolia prevention group-D (n = 15) and M. citrifolia treatment group-E (n = 15). By the end of the 28 weeks experimental period all the animals were euthanized, blood was collected by heart puncture. M. citrifolia treatment significantly (p<0.05) increased the anti-oxidant enzymes such as catalase, superoxide dismutase and significantly (p<0.05) decreased the lipid peroxidation activity when compared to NMU control group-C. M. citrifolia exhibited a preventive effect against anaemia, lymphocytosis and neutrophilia in group-D and group-E when compared to group-C. Biochemical analyses showed normal levels of enzymes of liver and kidney in M. citrifolia treated groups- B, D and E rats, whereas NMU control group-C showed significant (p<0.05) decrease in albumin and total protein levels. These findings indicate that M. citrifolia fruit juice did not show any hepatotoxic or nephrotoxic effects. It was concluded that the M. citrifolia fruit juice ameliorates the adverse effects of NMU carcinogenesis and could be useful to treat mammary tumours in humans and animals.