Lysimeters containing a soil monolith are generally recognized and used to study the environmental behaviour of pesticides in accordance with good agricultural practice. In order to validate the transfer of lysimeter data to the field/region scale, an extensive experiment was set up consisting of a 1-ha experimental field plot with five sampling tubes, a small plot experiment, 12 lysimeters, a measurement trench with 102 time domain reflectometry sensors (TDR) and 21 temperature sensors and a weather station. The leachate in the field and small-plot experiment is obtained using tensiometer-controlled ceramic plates. In addition to a mobile and an immobile test substance for determining mass transport, several water tracers are used over the three-year test period. A preliminary experiment for the further characterization of the test site and test systems was started in March 1997 with the application of 2,6-difluorobenzoic acid. Complementary laboratory studies are performed for process identification and generation of input parameters for simulation models.For active ingredients showing some leaching potential practice-like lysimeter experiments using radiotracer techniques play a central role. Lysimeter experiments using 14 C-labelled pesticides yield comprehensive information on the uptake of the respective compounds by treated and untreated rotational crops, the residue behaviour of pesticides in soil and leaching (1). With the addition of a wind tunnel erected over a lysimeter, a complete mass balance including volatilization and mineralization can be achieved (2). In 1990 the German government authorities published a guideline for lysimeter studies (3) required for those compounds which show a potential for leaching. As far as the dissipation in soil and residue level in treated plants are 152