In 1998 the UK Government, in conjunction with Supply Chain Initiative on Modified Agricultural Crops (SCIMAC, a cross-industry group representing the supply chain), established a large-scale programme to examine the impact of genetically modified herbicide-tolerant crops on farmland biodiversity. The trials programme lasted three years and was conducted by a consortium of scientists who were responsible to an independent scientific subcommittee. The background to the establishment of these trials is described and the political pressures facing the Government at that time and the reactions of key stakeholders are discussed.The field-scale evaluations were a pioneering project to examine the impact of genetically modified crops on farmland biodiversity. The exercise was one of the largest experiments ever carried out to measure the environmental impact of specific farming practices, and shows that robust results can be obtained, while the resultant database represents a wealth of ecological information on three important arable crops ? beet, maize and oilseed rape. It is clear that other such trials could be carried out to test other variables as part of a longer-term effort to steer farm management systems in ways more acceptable to society. However, in view of their costs (both to the UK Government and the technology providers) together with the adverse response by the environmental campaign groups, and the length of time needed to carry out the work, it seems unlikely that an exercise of similar scale will be undertaken in the near future.