This article examines the way in which public controversies affect regulatory science. It describes the controversy that unfolded in Europe around the use of the ninety-day rat-feeding tests for the risk assessment of genetically modified (GM) plants. This type of test had been criticized for almost two decades by toxicologists, nongovernmental organizations, and industry alike for its inability to capture the specific health effects of GM plants. But GM risk assessment experts showed great reluctance to move toward a more systematic use of other tests, such as chronic two-year studies or toxicogenomic techniques, and made sure that official guidance continues to recommend the use of the ninety-day rat-feeding study. The article shows that these tactics of standardization are a defining feature of regulatory science, and a resource for toxicity experts to defend their authority and credibility against competing expertises that arise during controversies.