200 rats were divided into ten groups: three that had part of their standard lab-rat diet replaced at varying levels (maximum 33 percent) with Roundup Ready corn that had been treated with Roundup in the field; three getting the same feed protocol, but with untreated Roundup Ready corn; three getting no GMO corn but tiny amounts of Roundup in their drinking water at varying levels; and one control group ate two-thirds standard lab-rat chow and one-third non-GMO corn. Each group contained 10 females and 10 males.

The two year study, is the longest independent, peer-reviewed research carried out on the potential health impacts of genetically engineered foods. The study shows that consuming even relatively low levels of (Roundup tolerant) GM corn or of Roundup itself, greatly increases levels of mammary tumors, kidney and liver damage, and premature death in laboratory rats.

By the 24th month, 50% - 80% of the females had developed large tumors compared to 30% in the control group. The researchers believe that long-term studies are needed to evaluate the safety of GM crops. Currently all GM food crops have been approved safe on the basis of 90-day feeding studies in mammals. In this two year study, the tumors and kidney and liver damage did not manifest until 120 days into the research.

The research was conducted by a team of scientists led by molecular biologist and endocrinologist, Professor Gilles-Eric Seralini, co-director of the Risk Quality and Sustainable Environment Unit at the University of Caen, France, who is an authority on studies into the health impact of GMOs and pesticides. It was supported by the independent research organization, CRIIGEN.

After this study was reviewed by the Russian government, they suspended all imports of Monsanto's genetically engineered corn.

Long term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize published in The Food & Chemical Toxicology Journal. Full report -  Summary.